Knowledge Base

AC: Alternating Current. The voltage varies constantly above and below zero in a 50 or 60 Hertz sine-wave. Used for power distribution because the voltage can easily be changed by a transformer.

Ampere (or Amp): The unit of electric current. An analogy would be the amount of water going over a waterfall.

Avalanche Diode: A type of semiconductor component that is normally open circuit until the voltage increases to the point where the device turns on and conducts current. Similar in operation to MOVs but do not degrade with use. Very reliable as long as they are used strictly within their ratings.

Brown out: A sustained under-voltage condition which is low enough to cause equipment malfunction. Most equipment can operate from 105V to 135V without serious performance degradation. A voltage lower than 90V will usually cause functional problems.

Capacitance: The effect where electric charge is stored. A capacitor is an electronic component made specifically to store electric charge. Capacitors are also used to differentiate between frequencies in applications such as crossover networks. A capacitor has high impedance at low frequencies (open circuit at DC) and its impedance decreases as frequency increases.

DC: Direct Current. The voltage is constant and does not vary. Used primarily inside electronic equipment.

EMI: Electromagnetic Interference. A general type of electric, radio or magnetic interference which is transmitted by conduction or radiation and can be of a very wide frequency range.

Hertz: The unit of frequency in cycles per second. Used to characterize anything from AC power (50 or 60 Hertz) up to cellular phone radio frequencies (Giga-Hertz)

Inductance: The effect where energy is stored in a magnetic field. An inductor is an electronic component made specifically to store energy by using a magnetic field. Inductors are also used to differentiate between frequencies in applications such as crossover networks. An inductor has low impedance at low frequencies (short circuit at DC) and its impedance increases as frequency increases.

Inrush:  The initial current that occurs when a motor or electronic equipment is first turned on. The inrush current is usually several times higher than the normal operating current. In the case of electronic equipment the initial inrush current occurs while the power supply charges up. Equipment with a large current draw can have an inrush current large enough to blow a circuit breaker or damage switches and relays.

Joule: The unit of energy (measured over time). One Joule is equivalent to the heat generated during one second when one Volt is driving one Amp around a circuit.

MOV: Metal Oxide Varistor. A disc shaped device which is normally open circuit until the voltage increases to the point where the MOV turns on and conducts current. Originally developed to suppress arcing on relay contacts but used extensively in shunt-mode surge protectors. Have a limited lifetime.

Parallel: Devices connected together so that the same voltage appears across all devices.

Power Surge or Voltage Surge: A short term over-voltage condition. Surges caused by lightning are very high power but extremely short in duration, lasting only for 20 to 50 millionths of a second. Surges caused by equipment switching and other sources are also extremely short lasting typically for less than a thousandth of a second.

RF interference: Radio Frequency Interference. Interference which is caused by radio signals.

Series: Devices connected together in a chain so that the same electric current passes through all devices.

Series Mode®: A brand of surge protector which uses a high-voltage device to prevent surges and transients from being passed to connected equipment.

Shunt Mode: A type of surge protector which uses MOVs, avalanche diodes or gas discharge tubes to conduct surge current to the neutral or ground wire.

Switch-mode power supply: The power supply inside electronic equipment converts the 120V AC to the DC required by the circuitry. A switch mode supply is smaller and lighter than a traditional power supply because it switches the current on and off very fast to generate the DC using a small high-frequency transformer rather than a larger traditional power transformer.

Toroidal Transformer: A transformer that is constructed around a ring of iron. This produces a more efficient, compact design with a tighter magnetic field.

Transient or Spike: Similar to a voltage surge in that they cause a very brief over-voltage condition, but are typically less energetic and less damaging.

Volt: The unit of electric potential. Voltage drives electric charge around a circuit. An analogy would be the height of a waterfall.

Voltage Regulation: A means of maintaining the equipment voltage at a constant level. Domestic AC power is distributed at a nominal 120V but this can vary depending on the load and other factors in the distribution. A voltage regulator adjusts for those variations to provide a constant 120V at the equipment.

Watt: The unit of power (continuously generated). One Watt is generated when one Volt is driving one Amp around a circuit.

Series Mode® surge protection operates in a fundamentally different way to conventional surge protection. Series Mode brand surge protectors block and contain surge energy and do not simply transfer energy between the wires of a branch circuit like all MOV based surge protectors. The term “Series Mode” is taken from the fact that the main protection element is in series with the AC. True Series Mode products do not use any components, sacrificial or otherwise, that shunt energy between the wires of an AC circuit.

True Series Mode surge protectors are the only ones that can safely be used on branch circuits without causing problems. Both conventional and hybrid* surge protectors operate by diverting surge energy to neutral and/or to ground instead of blocking and containing it. When installed at the service entrance, they can effectively shunt surge energy to the ground rod provided the protection components are rugged and reliable. However, when plugged into a branch circuit there may be several feet, or even hundreds of feet, of wire between the equipment and the ground rod.

The concept that surge energy can be directed to the ground rod through a branch circuit is flawed. Wire between the equipment and the service entrance (ground rod) has significant impedance to transients and surges, and only a part of that energy will take the preferred route to the ground rod. There are only two possible scenarios:

  1. Surge protectors which have MOVs between live, neutral and ground divert energy to both neutral and ground, and will contaminate both the neutral and the ground wires. The safety ground is the same as the equipment chassis and system ground, and surges diverted to ground will allow energy to flow in chasses and cables that join interconnected equipment. Dumping surge energy onto the system ground instead of blocking and containing it can result in high-voltage energy traveling along interconnecting cables and causing permanent damage to input or output stages.
  2. Surge protectors which have MOVs only between live and neutral divert energy only to neutral and do not contaminate the ground wire, but they are also not effective at completely protecting equipment. Not having any MOVs or other shunt devices to ground, they allow live and neutral to float with respect to chassis ground up to dangerously high voltage levels during a surge event. To use the technical term: they convert normal-mode surges to common-mode surges. Common-mode surges are normally only found at low, harmless levels on branch circuits, but when generated by such a piece of equipment they can be large and damaging. Power supplies in electronic equipment may be damaged, and equipment that has a common-mode filter on the AC power input will almost certainly be damaged. Common-mode filters often have capacitors from live and neutral to ground and these capacitors will be blown out by the large common-mode surges that are generated by this type of surge protector. It is extremely important that no other surge protectors which have MOVs to ground be plugged into the type of surge protector which has MOVs only between live and neutral. To do so will convert the protection back to the type which has MOVs between live, neutral and ground resulting in ground contamination exactly as per the discussion in (1) above. There is also the risk of blowing out the MOVs that are inside the second surge protector. This also applies to UPSs since most UPSs have MOVs between live, neutral and ground. Whereas it has been standard practice for many years to plug a UPS into a SurgeX in order to protect the UPS front-end, plugging a UPS into a surge protector which has MOVs only between live and neutral cannot be recommended for these same reasons. The SurgeX True Series Mode architecture effectively “disconnects” MOVs inside any equipment plugged into it but this is not the case with other architectures.

* Hybrid surge protectors using the term “Series Mode” may be a standard shunt mode product that has a low-voltage series section following it. These hybrid products produce the same problems cited above that standard shunt-mode products produce. Interestingly, if we look at how effective such a product is at handling energy the result is startling: Energy is proportional to the square of voltage, so if we assume a hybrid product is using MOVs to drop the normal-mode voltage down to 400-500 volts, then for a 6000 volt surge this would result in more than 99% of the energy remaining on the branch circuit. Almost all of the energy is being shunted by the MOVs and left on the branch circuit!

Further reading:
The Truth About MOVs (Metal Oxide Varistors)
Series Mode vs. MOV, Tony Keane

Series Mode; a brand of surge protector that uses high-voltage filtering techniques to block and contain surge energy and filter transients without dumping energy on the neutral or ground wires.

MOV (Metal Oxide Varistor); a device that operates like a voltage dependent switch. Below a preset voltage it is mostly off and above a preset voltage it is mostly on. Have been used since the 1970s in suppression and protection circuits.

Normal Mode; surges and transients that present a voltage between the live and neutral wires. (Sometimes also called differential mode.)

Common Mode; surges and transients that present a voltage simultaneously on live and neutral with respect to ground.

Branch Circuit; an AC circuit that originates at a breaker or distribution panel and feeds one or more outlets.

Service Entrance; the point at which the AC service enters a building from the power company. Usually accompanied by a breaker panel.

Ground Rod; A copper stake driven into the ground to provide a safety ground for the building. Usually provided at the service entrance.

* Series Mode is a registered trademark of New Frontier Electronics, Inc.

This application brief outlines how telephone, cable, & satellite services enter a building and how these services should be properly installed and connected to protect connected equipment from lightning & man-made surge and transient damage.

Service Entrance Surge and Transient Protection

Telephone, CATV, and Satellite Service

Telephone, CATV, and satellite services that enter commercial or residential premises are subject to surge and transient events in much the same way that the electrical service is. Direct and induced energy from lightning, as well as man- made surges and transients can find their way onto these services and subsequently into the building, damaging or destroying connected equipment. The most common form of protection offered by the service providers is in the form of a shunt mode device such as a MOV, gas discharge tubes, carbon and other arc-through block that is put in line with the transmission or phone line and connected to earth ground at a convenient place. The theory being that when a surge event occurs from lightning or some other source, the device will divert the energy to earth ground once a certain break-down or arc-over threshold is reached.

Electrical Service

A ground rod is required by code at the power service entrance of a building. It, along with the service entrance and building wiring, provide the first line of defense against lightning entering the building. It does so by allowing surge energy to arc-over from the service entrance and building wiring to earth ground thus limiting the maximum voltage seen inside the building to around 6000 volts.

Proper Installation of Services

For surge energy to properly get diverted to earth ground at the service entrance and protect connected equipment, it is imperative that ALL of the services enter the building at or very near the same place so they can ALL share a common earth grounding point. Unfortunately this is often not the case. It is quite common to see the electrical service enter a building at one location and be properly grounded, while the other services come in at different locations, usually dictated by convenience to, or ignorance by the installer. Once this happens, even if a ground rod is placed at the different service entrances or a less capable ground such as a water pipe is used, the rule of using a common grounding point for all services has been broken and problems will begin to occur.

Multiple Earth Ground Sources

Once different grounding points have been used in service installations, any surge current that gets properly diverted by the surge protection device enters the earth ground through a wire having some small amount of impedance. This impedance, although perhaps only a few tenths of an ohm when presented with many thousand amperes of surge current to divert, will cause the ground potential (what we normally think of as the zero voltage reference) to rise many hundreds or even thousands of volts at that point.

Shared Services

If all services entering the building properly share a common earth ground at the service entrance the phenomenon described above would have no effect on connected equipment. All equipment would reference the same ground point and there would always be near zero volt difference between all equipment during a surge event. Such is not the case when different ground references are used and a surge event occurs. The many hundred or thousand volt shift in the ground reference due to having different ground sources is now present between the equipment that shares these services. With a different ground reference potential between equipment that is otherwise connected together with low- voltage telephone, CATV, network, or audio wire, a completed circuit is created and the voltage that is present (usually several orders of magnitude greater than normal) causes damage or destroys the connected equipment. Line-level audio inputs/outputs on audio equipment, telephone PBX’s, and network interface cards are particularly susceptible to this type of damage due to the much lower voltage and current signals present in normal operation.

All-in one Surge Protection Devices

In recent years, all-in-one surge protection devices have found their way into the marketplace. These devices range from inexpensive power-strips to high priced units that connect to the power, telephone, and CATV/satellite systems and claim to protect the connected equipment from surge and transient damage from any and all sources. These devices fail in three critical areas to protect equipment and proved a clean source of power.

No Common Earth Ground

Because these devices use shunt mode technology in the form of Metal Oxide Varistors (MOVs) to shunt the surge energy to ground AND because they are not mounted at the service entrance, but rather sitting well inside the building and on a branch power circuit, they increase the problem of ground circuit contamination and ground loops. Any surge current that is successfully diverted to the ground wire inside the box must now travel some distance, usually several hundred feet through building wiring to the earth ground. This sets up that same ground-loop scenario as previously described, causing a voltage difference between connected equipment with subsequent damage.

Redirection of Energy

Anytime a significant surge event such as a lightning strike occurs it needs to be mitigated in the best possible manner before the majority of the energy can enter the building. If energy of any significant magnitude is allowed to enter the building though a service such as the power, telephone, of CATV/satellite system, the absolute worst scenario is close placement of these services at a central point of connection such as an all-in-one surge protection device. Large surges will arc over from one service to another damaging otherwise unaffected equipment.

Cross coupling of EMI/RFI

Once multiple services are brought into close proximity to each other, crosstalk through both high and low frequency signals occurs. The relatively large magnetic fields around the power lines will couple 60Hz AC noise into the telephone and RF signals paths and conversely, EMI and RFI from the satellite/CATV and high frequency data services on the phone line will find their way into the power and manifest themselves as audible noise, video degradation and so forth.

Installation Guidelines for Services

Common Entrance/Ground

All services such as power, telephone, and CATV/satellite, need to be brought into the building at nearly the same point so that they can all share the same common earth ground.

Proper Protective Devices

All services need to have proper surge protection devices installed by the service provider and connected to the common earth ground as described above.


Telephone service entrance is usually provided in the form of a telephone network interface at the demarcation point. These devices usually uses some type of shunt mode protection to provide protection from lightning and other sources or large surge events from entering the premises. This the placement of the telephone service entrance needs to be close the other services, including power so that the common grounding point can be used.


Cable television service entrance is often provided in the form of a junction at the demarcation point with a air- gap arc-over block. Again, the placement of the CATV service entrance needs to be close the other services, including power so that the common grounding point can be used for the surge protection device.


Satellite dishes need not be mounted on a rooftop and be turned into a natural lightning rod. Height above ground, unless for obstruction clearance or aesthetics is not critical for the operation of these devices. Satellite cable surge protection is often provided similar to CATV in the form or an air-gap arc-over block. The placement of the satellite service entrance needs to be close the other services, including power so that the common grounding point can be used for the surge protection device.

If a rooftop installation is mandated, consider routing the feed line inside metal conduit from the dish down straight down the side of the building with no turns and ground the conduit to the common service entrance ground. At the point the feed line enters the building bring it in at a 90-degree angle to provide a high impedance path to lightning.

This technique will not protect the feed horn, LNB or even the cable from damage due to direct lightning strike, but it will provide a reasonable defense against lightning entering the building by providing a path for the lightning to arc-over into the conduit and find a path to earth ground.

Most people are aware of the fact that electrical and, in particular, electronic equipment can be damaged or destroyed by lightning. This is a dramatic and probably the most extreme example of surge damage, and equipment definitely needs to be protected from lightning damage, but surges and transients can also be produced by equipment located inside a building.

Surges or voltage transients are abnormally high pulses of voltage that substantially exceed the normal operating voltage of a circuit. They are generally random in nature and may last anywhere from tens of nanoseconds (a billionth of a second) to a few milliseconds (a thousandth of a second). Advances in semiconductor processing technology, producing devices with smaller and smaller geometries, have also limited the ability of these devices to absorb transient energy. As such devices are designed into electronic equipment, the overall susceptibility of the system increases. With billions of dollars’ worth of equipment now being lost each year to the effects of electronic overstress, transient protection is no longer a luxury but a necessary part of system design.

Lightning Surges

There is absolutely no way to protect a piece of equipment in a part of a building that gets a direct hit by lightning! Usually, however, when a building or a utility pole gets hit, the equipment itself does not receive the direct discharge: the equipment receives a surge through the building wiring. Research has shown that because of arc-over at the service entrance and within the building wiring, the maximum voltage that reaches a 110V outlet is 6000V. The same research has also determined that the maximum current is 3000A. We now have an idea of what a surge protector must be able to reliably handle in order to protect equipment from lightning damage.

All parts of the USA receive a significant frequency of lightning strikes in a year, but there are regional variations. Being on a hill increases the risk of a lightning strike as does the geographic location of a building. A surge protector in a “hot” area such as Tampa, Florida may have to withstand more surges than a surge protector in Alaska. Thus, it is not just the energy of a surge that has to be considered, but the frequency of surges. Also, lightning is not only hazardous to equipment just when the strike is very close, the intense electric and magnetic fields surrounding a typical 20,000 amp strike will induce a voltage of around 2000V at a distance of 300 feet in just 3 feet of wire, and will still induce hundreds of volts at a distance of ½ mile! This is why equipment failures occur during a storm when no evidence of a strike is apparent in the immediate area. The accompanying graph shows the typical relationship of induced voltage to distance.

View Graph

Inductive Surges

When any piece of equipment that contains an inductive element, such as a motor, transformer or coil is switched off, what is called a “back-emf” is produced. This back-emf, which is caused by the collapse of a magnetic field, is the result of one of the most basic laws of electricity, Faraday’s Law of Induction. The voltage thus produced can be several times the original voltage applied to the inductive element before it was switched off, although the duration of these surges (or transients) is very short. Electric cattle fences are energized using this principle – a coil is repeatedly energized (often by a 12V battery) and then the current is shut off. The resulting back-emf can be several hundred volts, as anyone who has touched such a fence finds out!

Such inductively produced transients may not be as energetic as a lightning strike, but similarly damage and degrade electronic equipment when they find their way onto a circuit board. Inductively produced transients, which travel throughout building wiring, are commonly produced by air-conditioning equipment cycling on and off, by refrigerators, and by any other equipment containing motors or other inductive elements.

It is plainly desirable to protect equipment from lightning surges and from inductive transients produced within a building, especially if the equipment is very costly or is in an application which absolutely must remain operational. Protecting equipment with surge protection not only prevents catastrophic failures but also improves the reliability of electronic equipment by preventing degradation and premature failure of integrated circuits.

The SurgeX Impedance Tolerant® EMI/RFI filter is a technical advancement in the field of EMI/RFI filtering. Incorporated into the higher performance SurgeX® surge protectors, it offers real-world performance improvements over conventional EMI/RFI filters. This is in keeping with the SurgeX Series Mode technology, which offers the highest level of performance and reliability in the field of surge protection.

Conventional EMI/RFI filters are designed for the standard bench source and load impedance of 50 Ohms. This standard is used throughout the electronics and communications industries and has a historical legacy dating back to the early days of radio. However, this standard has no real-world bearing on the power industry – source and load impedances will, most likely, be anything but 50 Ohms. Therefore to design an AC power filter to operate with a 50 Ohm source and load impedance is an academic exercise with no connection to the real-world! At best, the performance will be mediocre in real-world applications, and at worst it can be detrimental to the overall performance of the equipment. Passive electronic filters (which include EMI/RFI filters) are designed for specific input and output (source and load) impedances, which are normally resistive. The input and output resistances are actually part of the filter design, and the desired attenuation characteristic is only achieved when these exact resistances are present. Now, take a real-world situation where the input and output impedances are anything but 50 Ohms, and are also not purely resistive: the whole characteristic of the filter changes. Computer modeling has shown that, not only does the attenuation characteristic fail miserably, but resonant peaks caused by input and output mismatch can actually increase the interference level at certain frequencies!

The SurgeX Impedance Tolerant® EMI/RFI filter has been designed with real-world power line impedances in mind and is tolerant of mismatch in source and load impedance, hence the term: Impedance Tolerant®. In conjunction with the Series Mode® technology surge protector, the Impedance Tolerant® EMI/RFI filter provides the most effective surge protection, power filtering and conditioning available.

The three-stage design provides safe, clean power in the following manner:

  • The SurgeX® Surge Reactor (6000 volt inductor) handles the heaviest surges and blocks 90% of the largest surges to be found on power lines. It also filters out transients produced by electrical switching and heavy equipment. (First stage of EMI/RFI filtering)
  • The SurgeX® Clamping board clamps all remaining surge and transient energy, and also provides the second stage of EMI/RFI filtering
  • The final third stage of power conditioning is provided by the Impedance Tolerant® EMI/RFI filter giving you the confidence of knowing you are supplying totally clean power to anything plugged into one of these SurgeX® products
Argentina 220 50
Australia 240 50
Austria 230 50
Bahamas 120 60
Belgium 230 50
Bermuda 120 60
Brazil 110 – 127, 220 60
Bulgaria 220 50
Cambodia 120, 220 50
Canada 120 60
Chile 220 50
China 220 50
Colombia 110, 220 60
Costa Rica 120 60
Denmark 230 50
Ecuador 120 60
Egypt 220 50
El Salvador 115 60
England 240 50
Finland 230 50
France 230 50
Germany 230 50
Greece 230 50
Hong Kong 200 50
Hungary 220 50
Iceland 220 50
India 220 – 250 50
Iran 220 50
Iraq 220 50
Ireland, Southern 220 50
Ireland, Northern 220 50
Israel 230 50
Italy 230 50
Jamaica 110 50
Japan 100 50
Jordan 220 50
Kenya 240 50
Korea, South 110, 220 50
Kuwait 240 50
Mexico 127 60
Netherlands 230 50
New Zealand 230 50
Norway 230 50
Philippines 110 – 115 60
Poland 220 50
Portugal 230 50
Puerto Rico 120 60
Russia 220 50
Saudi Arabia 127, 220 50 / 60
Scotland 240 50
Singapore 230 50
South Africa 220 – 250 50
Spain 230 50
Sudan 240 50
Sweden 230 50
Switzerland 230 50
Taiwan 110 60
Thailand 220 50
Turkey 220 50
United States 120 60
Venezuela 120 60
Vietnam 220 50
Virgin Islands 120 60

Ordinary surge protectors (and Uninterruptible Power Supplies) simply divert harmful surge energy from the hot line to the neutral and ground wires in a process usually described as “all 3 modes of protection”. The hot line is the only source of dangerous external surges since the neutral and ground are bonded together and fastened lo an earth rod at every service entrance. Unfortunately, this “3 mode protection” process diverts high-energy powerline surges directly into reference grounds in audio/video systems and delicate, low voltage datalines in computers. Computer datalines are particularly vulnerable because they use the powerline ground circuit for their reference voltage.

Sensitive electronic equipment should never use ordinary surge protectors, which divert surges to the powerline ground, because this will increase the likelihood of damage.

A Breakthrough in Surge Protection

U.S. Patents 4,870,534 and 4,870,528
In today’s work environment, the need for surge protection has become an essential fact of life. Your electronic systems, components and machinery all use integrated circuits that are vulnerable to performance disruptions or costly failure due to surge related events.

Two basic types of surges affect your equipment. They are officially classified as Type 1 and Type 2.

  • Type 1 surges, commonly caused by equipment such as copiers or appliances cycling on and off, affect equipment many times during the course of the day. Over time, Type 1 surges will cause circuitry – as well as surge protectors without Series Mode technology – to degrade and fail
  • Type 2 surge events typically occur during a lightning storm

Until recently manufacturers tried to protect your equipment by diverting the surge energy to the ground line of the electrical circuit in the building. This has proven to be dangerous for a number of reasons. But before New Frontier’s technology breakthrough, it was the accepted and routinely installed method. Now there’s a better way.

SurgeX offers dependable protection for both Type 1 and Type 2 surges. Its patented Series Mode technology is the only type of protection to qualify for the U.S. Government’s stringent A-1-1 Classification. It is the safest, most effective and most reliable known surge suppressor for any type of office or systems application.

For Your Computer

Whether your computer is used in the home or workplace, it is susceptible to daily surge events that could disrupt or destroy it. Something as common as the heater or air conditioner turning on and off can eventually take its toll on the integrated circuitry inside. SurgeX not only protects against these every-day surge events, but against catastrophic surges as well.

Other surge protectors try to solve this problem by sending surges to the ground wire of your electrical system. This can damage unprotected equipment in the same circuit. And what’s worse, this type of surge protector only works for a finite number of surges. Its circuitry decays with every surge event, ultimately leaving it useless – and your valuable equipment unprotected.

Vastly superior to conventional surge protection devices of the past, SurgeX patented Series Mode technology does not decay with use. And it keeps your ground wire free from contamination. This is especially important in digital networks that use the ground as a reference.

For Office Equipment

SurgeX provides round-the-clock protection for all your office and communications equipment. As mentioned, something as seemingly harmless as an electric letter opener creates small Type 1 surges. And just like computers, electronic office products fall victim to surge related events. For example, the key serve unit (KSU) in your office phone system can lose its memory from lightning discharges. Voice mail systems fail in the same way. SurgeX will protect you from surge-related events and help sustain the normal operation and longevity of your equipment.

For Audio-Visual Equipment

All sophisticated audio-visual products rely heavily on sensitive integrated circuitry for optimal performance. They’re vulnerable. And other electro-mechanical components can be affected too. Take video projectors, for example. Bulb failures are a common result of surge events. SurgeX protection from Type 1 and Type 2 surges will help extend the life of the bulb, protecting it – and your next presentation – from catastrophic failure.

Many government offices and major corporations have switched to SurgeX after realizing that there is no better way to protect hundreds of thousands of dollars worth of audio-visual equipment. By saving replacement equipment costs, procurement times and the associated hassle, SurgeX just makes sense.