There are 3 primary methods in common use for providing surge protection, where each method has certain advantages and disadvantages in their ability to provide surge protection.
These are:
• Silicon Diodes (SAD’s)
• Metal Oxide Varistors (MOV’s)
• Spark Gaps Diverters (SGD’s)
Comparison of common
technologies
Silicon Devices
The Silicon Avalanche Diode (SAD) devices include such devices as TransZorbs, Zeners, Sidactors, etc. They are typically characterised by a predictable low let-through voltage, a fast response time, a very low surge rating, and a very high cost.
Silicon devices typically have a lower clamping voltage and better clamping ratio for the same MCOV of a MOV device. For a 220-240V rated piece of equipment a let-through of 600V-1000V is desired.
As a MOV based device can adequately protect to this level, there is a strong argument as to whether the extra cost premium for silicon based protection device may be better spent on MOV based protection.
Comparison of SAD and MOV surge rating and let-through voltageAdditionally as most Silicon components on the market are generally in the 30-50V range, Silicon SPDs for mains voltage protectors are made up of series strings. If excessive lead lengths are used in the internal construction, the internal voltage drop due to the lead length can be excessive, thereby negating the advantages of using the silicon devices.
Silicon devices are well known for their low surge ratings. This is partially overcome by paralleling many devices together, but obviously with a corresponding increase in price.
Another well documented problem with Silicon based devices is that they are not robust – whereby exceeding their energy rating by more than 10-20% will give a 100% failure. With the series/parallel matrix of components, once one component is stressed and fails, a chain reaction will commonly occur, failing the entire device. MOVs and spark gaps on the other hand, are robust and can commonly exceed their surge rating by over 50%, and in many instances tested samples have exceeded their expected life by a factor of 2-4 times.
Many manufacturers of the silicon devices promote the high speed of the products, which in many instances can be misleading and irrelevant in the proper selection of Surge Protective Devices. Claims of speeds from 5ns to <1ns are commonly quoted.
Although some manufacturers will claim these fast clamping devices are a better product, close inspection will show that the poor installation practices and the layout of long internal leads of many of these products will often cause larger let-through voltages to be provided due to inductance.
An advantage of silicon is that is does not degrade and will last for a very long time. however, they will fail if the energy rating of the device is slightly exceeded. As the energy ratings of the silicon devices are comparatively low, there is a high risk that a failure will occur due to excessive energy of one impulse, rather than an excessive total number of smaller impulses.
To adequately test for the weakness of Silicon devices it is recommended to:
• Specifically ask for ratings of each individual protection stages and each individual modes in a single shot and 20 shot 8/20µs ratings
• Have certified test results for each mode, not just calculated or theoretical results
• Obtain the replacement cost of each protection mode
MOVs are generally well accepted in the industry as being the low cost, all round best performer.
It is important to install suitable surge rated which have been considered to suit the environment, geographical location and criticality of the application. If not sufficiently rated
high enough, one of the main disadvantages of the MOV can be the life.
MOVs have a life that is limited by the number and intensity of the transients that are diverted. The relationship of which is non-linear.
Well designed MOV products usually have excessive capacity to give a high single shot rating, not as this is commonly expected to be required, but to give a much longer length of life with the more numerous smaller impulses.
They will also usually have some form of an indication system that not only detects the complete failure of the SPD, but also a partial reduction of the internal capacity allowing the owner to replace the unit before the equipment is left with no protection, where the critical loads might possibly be damaged.
This redundancy aspect is an important consideration, and it will not be found in the low cost budget range of surge protection products that are commonly available in supermarkets and hardware stores.
With selection of the correct size SPD, a typical life of 10 years should be expected
Within this family are the spark gaps which are naturally ventilated air gaps, and gas arresters which have the gap enclosed and contain a low pressure inert gas to lower the firing voltage. Due to encapsulation, Gas Arresters normally have a lower surge rating, and are not commonly used for power Surge Protection devices.
Spark Gaps are predominantly promoted by European manufacturers, and are typically characterised by high surge ratings, much higher let-through voltages and long life.
The advent of the new Triggered Spark Gap (TSG ) by ERICO has been well proven to now offer the premium level of Surge Protection available for critical applications, when used as the primary surge protective component when used within the Critec TSG Surge Reduction Filters.
For their compact size, spark gaps have a surge rating that is difficult to beat, however the surge rating may actually be larger than required for phase protection.
The surge rating is high, as after the high firing voltage (3000-4000V), the arc voltage falls to approx 30V, meaning that little energy is dissipated across the device as most is diverted back to the source or to ground. This low arc voltage can however crowbar the supply, causing the mains voltage to be short circuited (follow-on current), blowing upstream fuses and removing power to the site. The follow-on current can cause premature aging of the gap, especially on high short circuit capacity supplies.
The new Triggered Spark Gap diverters have been engineered to deal with the problems associated with follow current in most instances, although still present a higher let through voltage to the load when used in a standalone installation. When used within the Surge Reduction Filters they provide the lowest let though voltage of any surge protective device on the market.
It typically takes 3000-4000V for the traditional spark gap to break down, this allows a very high voltages to be let-through to the equipment prior to the unit firing.
A common European solution is to use a downstream secondary MOV unit to take out the left over impulse, but co-ordination between these two devices can be difficult as the downstream MOV will start clamping at approx 400V, thus potentially not allowing the voltage to reach high enough for the spark gap to fire. (An Erico Technical Note is available on this subject.)
For the above reasons Erico do not recommend the use of spark gaps for uses other than within Surge reduction Filters or for N-E protection modes only. N-E modes often require high surge ratings, but as mains voltage is not present, follow on current is not a problem and the higher let-through voltage is the equipment’s highest withstand mode.
To adequately test for the weakness of spark gaps, it is recommended to:
• Play close attention to let-through voltage specifications, as most devices will be in the order of 3000-4000V
• Specifically ask for details on follow on currents, at the expected short circuit rating of the supply
• Spark gaps may also have difficultly complying with the UL1449 requirements, unless they are supplied in a suitable enclosure
The quest for increasingly robust point of entry protection has led to the development of the CRITEC TSG (Triggered Spark Gap), high energy surge protection device (SPD).
The TSG diverter provides similar let through voltages to MOV based products but with much greater energy handling capability. It also diverts more energy to ground through an enhanced clamping mechanism.
Because of its capacity to absorb greater energy loads than comparable technologies, and the speed with which the wave form is returned to normal mains voltage, the CRITEC TSG is an extremely efficient first line of defence against high magnitude power surges.
Remarkably, the TSG clamps voltages to less than fifteen hundred volts, while simultaneously diverting surge currents in excess of 100kA in magnitude, from within the confines of a standard 2M DIN profile.
Spark gaps have traditionally had two major short comings, high let through voltages and poor follow current performance. Both of these problems have now been addressed with the advent of the new CRITEC TSG.
The high let through voltages traditionally associated with spark gaps, are a product of the high initiating voltage required to form the arc, typically three to four thousand volts. The Critec TSG overcomes this by sensing the arrival of a transient and then utilising a triggering device to generate an initiation spark which ionises the region surrounding the spark gap electrodes. This allows the spark gap to trigger on significantly lower voltages.
Follow current is the term used to describe the effect of a diverter clamping the AC mains voltage. As spark gaps clamp transients to below mains voltage, problems can arise if the arc is not extinguished effectively. The TSG has eliminated this problem by lengthening the arc and splitting it into several smaller arcs. This causes the arc to rapidly extinguish, resulting in higher fault current capacities, minimising wear of the spark gap electrodes, and preventing the unnecessary activation of upstream fuses or circuit breakers.
The CRITEC TSG can be mounted on the DIN Rail adjacent to other devices without risk of damage to those devices from plasma venting, historically a problem with spark gap products. Additionally, there is no requirement to mount the diverter off the backplane.
The principal application for this device is as a point of entry shunt protector, where it may be used to provide “stand alone” protection for robust devices such as electric motors, air conditioning and lighting systems.
Alternatively, the TSG can be used as primary protection for more sensitive equipment by acting to divert the surge and allowing a downstream LC filter to more effectively provide higher levels of protection
Important SPD Specifications for AC Power Protection
After identifying the power distribution system to which the SPD is to be connected, one should compare the following performance aspects of competing products:
1. Maximum Continuous Operating Voltage (MCOV) - aim for a figure of at least 25% above the nominal supply voltage.
2. Suppression Voltage - for 240V systems an SVR of <400V for electronic equipment and <600V for electrical equipment is recommended. This may be provided by cascading several levels of SPD’s.
3. Surge Rating - approximately 100kA (@8/20µs) at Point of service entrance, and 40kA (@8/20µs) for sub distribution and branch circuits is recommended.
4. Indication Status - a progressive life status indication from 100%, 50% and 0% in corresponding internal surge material stages, is preferable to just a simple OK indicator.
It is recommended that customers ensure all specifications follow AS 1768-2003, ANSI, IEC and UL 1449 Edition 2 standards where applicable. This helps to avoid product specifications that may be written in a confusing or misleading manner.
Do I need to Protect all Modes?
An SPD may provide multiple "modes" of protection by using multiple protection devices connected to different points within the circuit. An SPD that provides protection P-E, P-N and N-E would be considered a three mode protection device, whereas an SPD which provides protection only between P-E, or P-N or N-E would be considered as single mode.
The number of modes that require protection will always depend upon the type of power system and the location of protection within the sites power distribution.Many SPD manufacturers will highlight the importance of response time of the SPD, and while this is important, this is not the single consideration which supercedes any other factors.
An SPD that offers a fast response time, generally implies that a lower clamping voltage will be the end result and therefore will be the better protector, however this is not the case as there are many factors which contribute to the
The critical loads being protected are not affected by the SPD's response time, but by the residual peak voltage and waveshape reaching it. The magnitude of this residual voltage reaching the equipment is a result of many aspects of the SPD design including the surge & voltage ratings, the internal wire size, shape and loop area, and the technology used as well as its speed of response. Of all these contributing factors, the speed of response generally has the smallest influence.
Standards bodies like UL, NEMA and IEEE advise that speed of response is not a valid specification for comparison of SPD’s and that the let-through, or the devices clamping voltage, should be used instead, because it includes speed of response and other more influential factors that affect an SPD's ability to adequately protect downstream equipment.
What Speed of Response is Required?
Many SPD manufacturers will highlight the importance of response time of the SPD, and while this is important, this is not the single consideration which supercedes any other factors.
An SPD that offers a fast response time, generally implies that a lower clamping voltage will be the end result and therefore will be the better protector, however this is not the case as there are many factors which contribute to the
The critical loads being protected are not affected by the SPD's response time, but by the residual peak voltage and waveshape reaching it. The magnitude of this residual voltage reaching the equipment is a result of many aspects of the SPD design including the surge & voltage ratings, the internal wire size, shape and loop area, and the technology used as well as its speed of response. Of all these contributing factors, the speed of response generally has the smallest influence.
Standards bodies like UL, NEMA and IEEE advise that speed of response is not a valid specification for comparison of SPD’s and that the let-through, or the devices clamping voltage, should be used instead, because it includes speed of response and other more influential factors that affect an SPD's ability to adequately protect downstream equipment.
Why do I need Status Indicators on my SPD?
Metal Oxide Varistors (MOVs) are ideally suited as voltage limiting devices due to their economical cost and their ability to handle large surge currents. However MOVs are a consumable item and exhibit an operational life which is proportional to the number and amplitude of surges and transients. MOV life characteristic is non-linear, so doubling of the surge rating provides a far greater length of life (typically 3-5 times) for the same size surge.
Indication and alarm circuits should be provided by such devices to signal when their protection status has reduced to a level at which replacement maintenance is required. It is preferable that such indication occur prior to total depletion of the protection to allow a replacement module or device to be installed without the equipment being left unprotected.
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