Protecting against electrical surges - 20 things you should know

All electrical installations are subject to a frequently distorted electricity supply. Uncontrollable anomalies such as lightning strikes and electromagnetic interference can and do distort the AC supply into buildings. Some can be catastrophic, destroying equipment from a single surge, while others can be too small to notice on their own, but have a cumulatively detrimental effect on equipment that inexplicably fails over time. But there are simple ways to deal with these challenges.

There are four main types of voltage surges that can disturb electrical installations by being superimposed on the rated network voltage: atmospheric voltage surges, operating voltage surges, transient overvoltage at industrial frequency and voltage surges caused by electrostatic discharge.

I spoke recently with Justin Barrett, Product Manager - Low Voltage Final Distribution at Schneider Electric, to find out how to effectively deal with these detrimental surges and protect electrical installations.

Barrett listed a number of simple purpose-built products can be installed into electrical switchboards to deal with these anomalies: “Voltage limiters are used in MV/LV substations at the transformer output, and in IT earthing schemes. They can run voltage surges to the earth, especially industrial frequency surges.

“LV surge arresters are very different devices, as far as technology and use are concerned. LV surge arresters come in the form of modules to be installed inside LV switchboards. There are also plug-in types and those that protect power outlets. They ensure secondary protection of nearby elements but have a small flow capacity. Some are even built into loads although they cannot protect against strong voltage surges.

“Low-current surge arresters or overvoltage protectors protect telephone or switching networks against voltage surges from the outside (lightning), as well as from the inside (polluting equipment, switchgear switching, etc).

“Low-current voltage surge arresters are also installed in distribution boxes or built into loads.”

To assist electrical contractors better understand electrical surges and how to mitigate them, Barrett furnished Electrical Solutions with answers to 20 commonly asked questions, which are listed below:

1. How does a surge arrester work?

A surge arrester is a device that is intended to limit transient overvoltages on an electrical transmission line and divert current peaks. It applies to all devices whose function is to limit the amplitude, or size, of a transient overvoltage to a sufficiently low enough value that it does not present a danger to the electrical installation and switchgear it passes through.

2. Why combine a surge arrester with a disconnector?

Combining a surge arrester with a disconnection device offers the advantage of isolating the surge arrester from the rest of the electrical installation should the surge arrester go into short-circuit due to an overvoltage situation containing too much energy (eg, direct lightning strike on the building), or a DC (under 50 Hz) installation fault (eg, broken neutral, phase inversion).

The disconnector also ensures an ‘end-of-life’ indication - a ‘toggle down’ that makes it impossible to reset signals, requiring the surge arrester to be replaced. Note that the disconnector’s breaking capacity must be greater than or equal to the I_sc of the installation.

An integrated disconnector/surge arrester removes the possibility of an error when selecting an external disconnector; it simplifies wiring and improves the surge arrester’s effectiveness by having reduced connection lengths to the supply network.

3. How do you choose between fixed and withdrawable surge arresters?

A surge arrester is not a consumable protective device like a fuse. If it is correctly selected and installed, its service life is similar to that of the other devices installed in the electrical switchboard.

The use of a plug-in cartridge surge arrester is only of interest for tertiary or industrial installations where electrical installation maintenance procedures are required; for example, when testing the earth connection resistance and for which the network of earth connections must be isolated from the rest of the electrical installation.

4. How do you identify an out-of-service or faulty surge arrester?

The IEC 61643-1 international electrical standard imposes a requirement of ‘end-of-life indication’ for surge arresters that use varistors. On the Schneider Electric Quick PRD surge arrester, for example, in-service and end-of-life indications are included on the front face.

When the indicator is white and toggle down, the surge arrester is ‘out-of-service’. It can be put back into service by switching the toggle up.

When the indicator is red and the toggle down, the surge arrester has been destroyed. To be sure of this, the toggle should be moved up into the ‘on’ position. If the toggle falls back down, the cartridge of the surge arrester must be replaced.

5. How does an installation determine surge arrester effectiveness?

The shorter the connections between the mains network and the surge arrester terminals (ie, the ‘50 cm rule’), the more effective the protection provided. This is the parameter that has the most influence on the surge arrester’s capacity to ‘absorb’ a lightning current by limiting the total residual voltage, as ‘seen’ by the protected loads, up to a maximum value. This voltage must be less than 1500 V, since above this limit, the load is destroyed.

Let’s use the Domae Quick PF surge arrester as an example. To ensure that the total residual voltage ‘seen’ by the load is less than 1500 V, is ready to wire and for the ‘50 cm rule’ to be respected, the device integrates a double-entry earth terminal to allow direct connection of the earth to the surge arrester downstream of the earth terminal-block and a mains network connection accessory.

For a surge arrester installed using a 1 m long copper cable, the cable, when subjected to a 10 kA lightning current for 10 microseconds, will have a potential difference of 1000 V between its two ends (Lenz’s law U = L dI/dt). These 1000 V add to the 1500 V maximum potential difference across the surge arrester terminals (U_p protection level). The sensitive load will therefore ‘see’ 2500 V. It will consequently be destroyed as its impulse withstand voltage is only 1500 V.

6. How can a surge arrester be installed with guaranteed safety?

The most important role of a surge arrester is to divert overcurrents to earth as soon as an abnormally high voltage appears across its terminals. This diversion is only required for a very short time - in the order of a hundred microseconds.

The surge arrester is not, however, a universal protection device. When subjected to constraints in excess of its earth run-off capacity, or when subjected to a continuous overvoltage, due to a broken neutral or a phase-neutral inversion for example, it will be destroyed.

The consequences of such destruction may cause severe damage if the surge arrester itself is not protected by a protective device (also referred to as an ‘end-of-life disconnector’). This protective device ensures the security for the rest of the electrical installation.

When a surge arrester has been destroyed, the equipment is no longer protected against overvoltages, so it is vital that the surge arrester and the cartridge of the surge arrester be replaced as soon as possible.

7. Does the installation of a surge arrester stop nuisance tripping?

The installation of a surge arrester does stop nuisance tripping, given that overvoltages come from the electrical supply network.

Diverting the lightning current as far upstream as possible in the electrical installation also avoids the current being propagated along the various electrical distribution cables within the building.

The electrical equipment is thus protected against conducted effects (via the cables) and induced effects (electromagnetic fields created by impulse currents).

8. Is a surge arrester necessary if the earth connection is very good?

It is not the quality of an earth connection that is of most importance for efficient protection, but having earth connections at the same potential and good equi-potential bonding (referred to as ‘extraneous conductive parts’, which include metal frames, reinforced concrete steel frameworks and all conductive structures).

The earth resistance value (earth impedance) must comply with electrical installation standards such as IEC 60364. No particular value is recommended if a surge arrester is fitted in an electrical installation, unless the building has a lightning conductor. In this case, the earth impedance must not exceed 10 Ω.

9. What are the consequences if the surge arrester is not connected to earth?

For common-mode protection, the surge arrester does not function correctly. For differential-mode protection, the surge arrester does not need to be connected to the earth since the surge arrester ensures a level of protection between phase and neutral.

10. Does a UPS protect the loads against the effects of lightning?

A UPS protects loads against brief or extended interruptions to the electrical supply. It does not protect loads against atmospheric overvoltages.

To improve supply continuity to loads while avoiding the UPS from being destroyed by atmospheric overvoltages, a surge arrester should be installed upstream of the UPS.

11. What is the average life of a surge arrester?

Since lightning is a random phenomenon, an exceptionally high transient voltage surge may exceed the capacity of the surge arrester and destroy it. Otherwise, they are designed to have an average life span that is equivalent to that of the electrical installation, but do progressively deteriorate over time due to the successive overvoltages they eliminate.

12. Do surge arresters provide complete protection?

A surge arrester efficiently protects an installation if its capacity is not exceeded. Unfortunately, complete protection does not exist because lightning is a random phenomenon and an exceptionally high transient voltage surge may exceed the capacity of the surge arrester and destroy it. Fortunately, withdrawable cartridge surge arresters are quick and easy to replace to provide protection again.

13. What if the installed surge arrester capacity is lower than what the risk assessment recommends?

A 20 kA surge arrester will provide the same protection as a 40 or 65 kA arrester; however, it will need to be replaced more often in comparison to if a higher rating was installed. If a risk assessment recommends a higher capacity (eg, 40 kA), then the probability of exceeding the lower capacity (eg, 20 kA) is higher.

14. What happens if a disconnection circuit breaker isn’t combined with a surge arrester?

The continuity of supply is no longer ensured, since the incoming circuit breaker for the installation will disconnect if the surge arrester short-circuits due to its capacity being exceeded.

15. What are the consequences of not replacing a faulty surge arrester?

The loads downstream of the surge arrester will not be protected when the next transient voltage surge occurs.

16. What could happen if the surge arrester is poorly connected?

A poorly connected surge arrester does not run the lightning current off to the ground efficiently.

17. Can surge arrester protection be conducted in three levels of cascading?

Yes, however the third level does not provide any extra protection because the incoming surge arrester is the primary diverter. A typical example of this would be 65 kA incoming protection + 40 kA secondary protection + 8 kA final protection.

Secondary protection is only needed if the incoming surge arrester has a U_p >1.5 kV and/or the distance between the incoming surge arrester and loads is >30 m. This is to make it compatible with the impulse withstand voltage of the equipment to be protected. Secondary protection is to be installed as close as possible to the load.

18. Can surge arresters be used on a DC current network?

Yes, however the surge arrester’s life span is reduced compared to operating on an AC network. In this case, withdrawable surge arresters are recommended.

19. What could happen if the surge arrester’s cable size to earth is too small?

There is a risk of cable deterioration over time, or even destruction, if a very high current flows through it.

20. Does the connection to earth have to be as short as possible?

Yes, for efficient protection the connections must be as short as possible (<500 mm from phase or neutral to earth). This is because all cables through which an impulse current flows have a voltage that is added to the U_p (voltage protection level) of the surge arrester. Therefore, loads downstream of the surge arrester are at risk of damage if this is not followed.