Frequently Asked Questions
- Should AC power Surge Protectors be connected phase to ground or phase to neutral?
- What are the SAD, MOV and GDT used in surge protectors?
- What is the significance of the Maximum Continuous Operating Voltage (MCOV)
- We have good grounding, so why do we need surge protection?
- What is the significance of redundancy of Surge Protectors for strategic or remote sites?
- Why are some surge protectors more expensive than others I have found?
- We do not have any problems with lightning, so why do we need surge protection?
- Why are some sites more vulnerable to lightning than others?
- A site cannot survive a direct strike, so why bother?
- What is the difference: multiple small MOV's in parallel and one using large block MOV components?
- What's a multi-stage hybrid in a communications or data surge protector?
- Why do I need to protect communications lines, the telephone company does that?
- Full protection is too expensive so why should I bother?
- What is the different function of a surge protector versus and filter.
- If the surge protector diverts the surge to ground, then it is safely "gone".
- All surge protection warranties are the same aren't they?
- What does Response time mean?
- What does Joules mean when talking about surge protectors performance?
- What is the benefit of a cascade or hybrid surge protector?
- What is the significance of a low impedance earth system at a site?
Q: Should AC power Surge Protectors be connected phase to ground or phase to neutral?
A: There is a practice by some surge companies to provide protection modules that are to be installed between phase and ground.
This is related to power transmission "arrestor's" that follow this practice because there is no neutral. In power distribution systems, the load is across the phase to neutral, so connecting the protection to ground, then from ground to neutral will effectively double the let-through voltage (LTV) to the equipment being protected.
Q: What are the SAD, MOV and GDT used in surge protectors?
A: These are the acronym or initials of the individual component technology commonly used in surge protectors. All technologies are simply a "Voltage Sensitive Switch". This means that at some line voltage, which is symptomatic of a surge, will cause the component to change state from a high impedance device to a low impedance path.
- SAD = Silicon Avalanche Diodes
- MOV = Metal Oxide Varistors
- GDT = Gas Discharge Tube
Q: What is the significance of the Maximum Continuous Operating Voltage (MCOV)
A: Surge protectors fail for one of two reasons; a surge that exceeds the current capacity of the unit or a swell in the line voltage supplied by the utility. Surge protection components have a voltage level at which it changes state to a low impedance path.
This is the MCOV. The component MCOV selected by surge manufactures for their products is typically 1.15 the nominal service voltage, as the power companies usually promise AC power will be nominal +/- 10%. In developing countries, the utility power can swell, for short periods, way above the +10% value. When this happens, the surge protectors conduct and start to carry line current, heating up and going into thermal runaway, until failure. This means that the surge protection on a site fail because of the utility power swells, unrelated to surges, and is often not covered by some warranties.
A higher MCOV generally implies a higher let-through voltage. Some manufactures use large block MOV components with higher MCOV's, but the large block MOV's used have a flat performance curve, which means the let-through voltage (LTV) is superior to most MOV's with lower MCOV's. This means that such products not only survive large surges including abnormal utility power swells and keep protecting your sites.
Q: We have good grounding, so why do we need surge protection?
A: A good site grounding system is very important. Every piece of metalwork, the tower, surge protectors and even the batteries are connected to the grounding system.
The surge protectors perform three functions:
- Divert or shunt damaging surge currents on the AC or Communications lines into the grounding system, to be dissipated safely
- When the grounding system potential is elevated, due to a direct or nearby strike, the surge protectors will equalize all copper lines at the site, to the ground potential (equi-potential) thus preventing "flash-overs"
- When the grounding system potential is elevated, due to a direct or nearby strike, the SPD diverts a portion of the current "off-site" to the lower earth potential at the distant power transformer.
Q: What is the significance of redundancy of Surge Protectors for strategic or remote sites?
A: When a surge protector fails in one or more of it's modes, the risk of catastrophic damage is greatly increased in the event of another strike. When a surge is large enough to activate a safety fuse in a surge protector, it is generally during a storm, not at the end.
Scientists have shown that a lightning event, known as a FLASH, consists of multiple STROKES of energy down the same channel. The individual surges are more than 10 ms apart with a global median is 3.2 strokes in a flash. This means there is a risk of a second surge down the same channel, to the same site, milliseconds later, that will damage the unprotected site, because all or part of the surge protection has failed.
You cannot afford to take this risk at a strategic site. If it were a remote site it could takes days before a technician can get to a site to replace a failed surge protector.
If the surge protector selected has built in redundancy, the site will still have substantial protection when some portion has failed. These products are common and inexpensive. Redundancy in the Surge Protection can significantly elevate the "Probability" of the site surviving a direct strike.
Q: Why are some surge protectors more expensive than others I have found?
A: There are surge protectors and then there are surge protectors. You get exactly what you pay for.
Low cost devices generally have poor protection performance, no safety features, and a warranty that reflects the manufacturer’s confidence in their own products. In addition, the technology used and the amount of surge current capacity will effect the unit price. Some companies will make erroneous claims about their product’s performance, surge current capacity or technology so that they appear to be as good as the professional products. They do this knowing that the customer will not or cannot verify their claims. There are a number of issues to consider when reviewing a surge protector’s price:
- Is the product's performance verified by independent testing such as UL 1449 3 edition tested and listed.
- What technology is being offered?
- Is the product a multistage hybrid, in the case of a communications or date SPD?
- Does the product have full redundancy?
- What is the surge current capacity?
- Does the warranty reflect the manufacturer’s confidence in their own product?
In the case of AC power SPD's, have a device that is UL 1449 3rd edition tested and listed is critical. A simple, published, unconditional, long term warranty ensures an installed, low life time cost of site protection. When these issues are figured into the price equation, the actual unit price of a professional surge protector is very competitive.
Q: We do not have any problems with lightning, so why do we need surge protection?
A: Some areas of the world experience less lightning than others. Wireless sites located on high areas with steel towers, are known to focus random lightning to the site.
Low frequency of lightning in an area merely reduces the probability of a direct strike to the site, but does not diminish the amount of energy and damage caused when a strike does occur. The profusion of expensive, delicate and fragile electronics in all aspects of our lives, connected to AC power, communications and data circuits makes the use of surge protection devices imperative.
Q: Why are some sites more vulnerable to lightning than others?
A: There are a number of factors to consider:
- The lightning frequency in the area.
- The height of the site relative to other structures in the area.
- The deterioration of the physical lightning protection system (earth resistance), due to corrosion and stress?
The attachment point of lightning is random in any given area. However, there are numerous published papers that discuss the influence of tall structures on the randomness of strikes. This area around a tall structure is know as the "area of influence" and is directly related to the height of the structure. Therefore a tall tower located on the tallest site in an area, will be struck in preference to any other location, within its "area of influence".
The earth resistance at a site, is a critical part of the lightning protection system in conjunction with the install surge protection devices.
The earth system is a combination of ground rods, copper wire and bonding as specified in the NEC article 250. The ground rods are typically copped clad steel rods, that begin to corrode the day the are driven into the ground based on the friction between the "dirt" and the rods.
As the earth resistance increase, so does the risk of lightning related damage at a site.
Q: A site cannot survive a direct strike, so why bother?
A: This is not true! There are about 5,000 cellular towers in Florida (USA), alone. Florida having a lightning density in excess of 5 flashes per square kilometers per year, each is struck between 1 and 6 times annually.
The industry understands the relationship of the surge protection devices and earthing and designs their protection systems to survive direct strike successfully, otherwise they would be spending millions each year replacing damaged equipment.
If you have a surge protector that has a surge current capacity of 40 kA, then you have a 55% percent probability of surviving a direct strike to the site. Using surge protectors with insufficient surge current capacity and the lack of understanding the damage coupling mechanism, creates the myth that your site cannot survive a direct strike.
A surge protector with 150 kA of surge current capacity is capable of surviving 97% of all direct lighting strikes. This is not merely a claim. There are thousands of sites in the SE USA that have survived multiple direct strikes for years and not sustained damage.
Q: What is the difference: multiple small MOV's in parallel and one using large block MOV components?
A: MOV's are solid state devices that can be considered as a simple electronic switch. They change their characteristics with stress. Generally an MOV will deteriorate if it is frequently diverting currents that are >70% of their surge current capacity. With enough stress they will eventually fail short circuit and may activate a safety fuse or burst into flames.
MOV's in parallel circuits will never equally share current. This is due to the 5 - 10% tolerances of the individual components used and in various aspects of their performance. This means that if a product has 10 MOV's connected in parallel that are individually rated at 10 kA to make a 100 kA device, you can expect one component to be carrying the majority of the surge current.
This can be caused by a characteristic such as a component that is at the lower end of it's MCOV tolerance, or a component that has a slightly faster response time. A surge current of >7 kA will eventually cause the one component to fail which result in the total loss of protection or the systematic loss of individual components as they fail, one at a time, in the string.
Single large block MOV's are typically rated at 90 kA or more. Repetitive surges of 63 kA or less, will have zero stress effect on the component. The use of large block MOV's plays a key role in the long term reliability of the surge protector and warranties offered by the manufactures.
Q: What's a multi-stage hybrid in a communications or data surge protector?
A: All surge protectors are designed to minimize the let-through-voltage, (LTV), to the equipment being protected, however, with all surge components the LTV increases with the amount of current in the surge.
On low voltage circuits such as communications and data, with logic levels of <6 volts, a simple single stage SPD will have an elevating LTV based on the current in the surge, that can cause fatal damage. A multi-stage hybrid uses a robust component in a primary stage, for surge current capacity, followed by other components whose job is to control the output of the previous stage.
The last stage, therefore, only has to handle the maximum output of the previous stage.
In this way, a single product incorporating multi-stages can control the LTV precisely, irrespective of the current in the surge.
Q: Why do I need to protect communications lines, the telephone company does that?
A: There is a misconception that a gas-tube protection device, installed by the telephone company, will be adequate protection for electronics. The gas-tube is installed as a safety measure to prevent fires or injuries to persons that may be in contact with the line, in the event of a surge. The gas-tube is not expected to protect the electronics connected to the line. A typical E1, DSL, ISDN or T-1 circuit has logic levels of 3-6 volts, connected into the heart of the base station. The let-through voltage (LTV) of a gas-tube is typically 1.5kV. and will destroy sensitive electronics.
Q: Full protection is too expensive so why should I bother?
A: Obviously economics plays a key role in any system installation. There are two decision factors in determining the amount that should be spent on protection.
- The STRATEGIC importance of the site such as a State wireless communications site responsible for all the Police, Fire, Rescue and other emergency services is critical. If the site were to fail, the consequences could be quite catastrophic preventing the various agencies from functioning. This would be considered a STRATEGIC site.
- The VULNERABILITY of the site to lightning strikes such as a radio site, with a 400' (125m) tower located on the highest point in the area, then intuitively it is reasonable to assume that lightning would strike this tower, in preference to any other location in the area.
- The lightning FREQUENCY in the area of the site.
Full protection is one of the most inexpensive insurance policies you can buy. The cost of system replacement, maintenance technician time, loss of customer satisfaction in a competitive environment and "critical systems downtime" is far more expensive than proper protection. Many SPD manufacturers have product lines with many options and features that would suit your budget. Models without UL 1449 3rd edition tested and listed, in the case of AC Power, or multi-stage hybrids, in the case of data and communications are at a lower cost that may sacrificing protection levels.
Q: What is the different function of a surge protector versus and filter.
A: Surge protectors are designed to deal with the HIGH energy, low frequency surges, whereas filters are designed to protect against LOW energy high frequency surges, often referred to as electronic "noise"
Q: If the surge protector diverts the surge to ground, then it is safely "gone".
A: The earth acts as a capacitor when a surge or lightning strike is directed to ground. The energy is being dissipated into the ground uniformly and the time that it takes to dissipate is a function of the resistance of the soil at the site. When ground potential is elevated, everything connected to it is also elevated. Any metallic connection to the site, from another location, will have a different ground reference and can flash-over. Surge protectors divert/shunt surges and equalize all potentials at the site to the elevated ground potential, preventing "flash-overs and damage.
Q: All surge protection warranties are the same aren't they?
A: They definitely are not. Many have lots of fine print with lots of conditions. Others only cover poor workmanship or faulty components. Then there are the ambiguous statements such as "lifetime warranty" without any definition of who's lifetime is being considered. Is it the component manufacturer’s expectation of their product, the lifetime of the buyer, the lifetime of that particular surge protector model ? You will only find out when you try to make a claim.
Q: What does Response time mean?
A: A surge will cause the surge protection components to change state from a high impedance device to a low impedance path. This is the "electronic switch" function. The time it takes to transition is the RESPONSE time.
Obviously this is a critical part of the performance of a surge protector and it will manifest itself in the measured let-thru-voltage of the complete surge protector. However, the response time of an individual component used in the SPD is not indicative of the response time of the complete surge protector.
For instance, an SAD component may have a peco-second response time, but when installed in a circuit with other SAD's in a matrix to increase surge current capacity, the response time of the SPD is increased above the individual components performance.
Specifying the response time for a surge protector is foolish as it cannot be verified. It is far more important to specify the let-thru-voltage with a standard test pulse, current and voltage. The damaging mechanism of a surge which is a combination of current and voltage, is the voltage component breaking down the dielectric strength at which time the current will cause the damage.
Q: What does Joules mean when talking about surge protectors performance?
A: The formula for Joules is simply "watts / seconds" This then refers to the amount of energy the unit can withstand for a given period of seconds, without failure. It would be more appropriate for a resistor or electric heater. As watts is a result of current times amps, and in the case of a surge, neither is known, or possibly Mv & kA, then it is not a useful gauge of a surge protector or component's capability. The more significant values to asses a surge protector is the "surge current capacity", let-thru-voltage as tested using the 8/20 u/s pulse an if it carries a UL label.
Q: What is the benefit of a cascade or hybrid surge protector?
A: All surge protection components have a performance were the let-thru-voltage increases with the current in the surge. Therefore, if you install a single large surge protector at the entrance to a building, or a single device on a data or communications line, then the let-thru-voltage, (LTV), to the equipment that you are protecting, is a variable related to the current in the surge.
This can mean LTV's in excess of 2kV can get through to your equipment. This is also true for a communication or data line, some of which operate at 5 - 50 volts. In the case of AC power, it is prudent to have a "Primary" surge protector on the mainelectrical service panel, then a secondary surge protector on a sub panel or at the point of use at the equipment to be protected.
This will remove the "uncontrollable" performance of your protection system and regulate the LTV. In the case of communications and data lines, it is practical to have multiple stages in a single small device that perform this function, so that the LTV is typically 20 - 200 volts depending on the circuit and surge protector chosen.
Q: What is the significance of a low impedance earth system at a site?
A: There are numerous schools of thought as to what constitutes a good earth. Telecommunications companies specified a 1 - 5 ohm earth, while some other industries have opted for a 20 ohm earth. There are a number of key factors in play that have to be considered when determining an acceptable earth impedance at a site.
- The resistance dictates the speed at which the energy is dissipated into ground. The objective is to minimize the time the site is "charged" due to the elevated earth potential. An extended time "charge" time will allow the energy to find one or more areas to break down the dielectric and cause flash-overs"
- Does your company or organization have a specification, if so, that is what you MUST strive to attain.
- Is this a "manned" site? The earth impedance in such a case is an integral part of the safety system to prevent injury to people at the site. Then the earth resistance must be 25 ohms or better to conform to the requirements of NEC.
- A simple rule is that if the site has electronics that are to be protected, then the lower the resistance the better.