Where to install a surge protection device - single phase domestic / light commercial supply

A surge protection device can be installed in two locations:

1. The consumer unit. This is the easiest to install but it takes up two spaces which may not always be available.

2. The isolator switch. This is more difficult to install as it involves having to remove the big fuse but it avoids taking up two spaces in the consumer unit.

Does anybody have any more comments or advice on the best location?

Proteus sells an attractive looking isolator switch with a 2 module SPD although it is a bit pricey compared with SPD modules for consumer units. Has anybody got experience of it?

Who actually owns an already installed isolator switch? Is it National Grid, the utility company, or the property owner?

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  • Wylex (and doubtless others) sell an SPD/isolator in a 4 module casing (REC4).

    Normally, an isolator is fitted in the consumer's tails so it belongs to the consumer, but it would be at the supply end so in a cabinet if there is one.

    I raised the question of positioning of an SPD at a NAPIT event in the spring. IIRC, somebody had mentioned that protection is provided only for a finite length down the load side. It follows that SPD protection is best placed in (or immediately adjacent to) final distribution boards rather than at the origin.

    If I have misunderstood the situation, please let me know.

  • I raised the question of positioning of an SPD at a NAPIT event in the spring. IIRC, somebody had mentioned that protection is provided only for a finite length down the load side. It follows that SPD protection is best placed in (or immediately adjacent to) final distribution boards rather than at the origin.

    What is the best way to install a SPD in a consumer unit? It slots into place on the DIN rail but there's more to it than that...

    Should it be physically located next to the main switch or is there anything wrong with installing it at the opposite end?

    Is it a good or bad idea to install it on the opposite (neutral terminal) side of the main switch to the MCBs / RCBOs?

    Some SPD modules connect their live terminal to a busbar but others connect with short lengths of wire. Where should the live wire go if there already is a live wire to RCDs and the end of a busbar for RCBOs inserted into the live output terminal of the main switch? It's a tight squeeze for a second live wire to the SPD!

  • The positioning of the SPD is based on ensuring that its "leads" are as short as possible, because the inductance of the wires for a rapidly rising voltage like a transient can be very high. So while the voltage across the SPD gets reduced to 1.5kV say, the voltage at the other ends of the leads only gets reduced to 2kV say, due to a 500V inductive drop along the leads.

    So what constitutes a part of the SPD's "lead"? Think of the path a normal or earth leak current takes through the L&N or L&E wiring system. Any part of the wiring of the SPD which isn't part of that route constitutes the "leads" of the SPD - it's a conductive path that will have a big voltage drop across it which is lost to the circuit it's supposed to be protecting.

    With that in mind, you have to consider which parts of the bus and N/E bars don't form part of the circuit. So if you have a breaker next to the main swich and an SPD at the far end of the bus bar (with connections to the N and E bars arranged similarly) then you're effectively adding the length of the bus bar plus the length of the N or E bar to the SPD lead length. So ideally the SPD wants to be positioned between the main switch and all the breakers. Similarly the SPD N on the N bar should be in position 2, where position 1 is the N from the main switch and positions 3+ are for the circuits. Ditto the E bar.

  • I think that you need to read section 534. It makes it clear that the leads to the SPD need to be as short as possible. If they are placed in a DB, that means one side or the other of the main switch.

    One should also take account of manufacturers' instructions.

  • In theory yes.

    One could argue that any interconnecting wires should not exceed the length of those supplied with the SPD by the manufacturer. It can be assumed that the manufacturer has tested the SPD and it works to spec with the wires supplied with it.

  • Yeah, but shorter is still better. The spec for the SPD will say something like "with an 8μs rise-time spike, the voltage across the SPD won't exceed 1kV".  With 0.5m leads, that increases to 1.5kV say. What if the rise time of a transient turns out to be 6μs instead? Then the spike may exceed 1.5kV and thus exceed the category II (1.5kV) rating of all those LED bulbs in the house.

    Keeping the leads as short as possible, rather than to just whatever the manufacturer supplied, will (in a small way) reduce the probability of a spike which end-user equipment can't deal with.

  • Take into account that most electronic devices are connected to the CU with significantly longer conductors than those between the main switch and the SPD. Notable exceptions are kWh meters and timeswitches installed inside the CU.

  • Yes but also note those longer conductors are not in the high current 'crowbar' path trying to shunt the surge, just in the series path transmitting the remaining voltage that was not shunted.
    That is the SPD, its tails and all the cable back to the origin of the surge. Ideally only a kV or so is dropped across the SPD, and we take our 'clean' supply for the sensitive loads from as close as we sensibly can to either side of the SPD body. The extra kVs piling up along the wires outside are someone else's problem ;-)

    Unless of course the surge is generated at the load end of things, in which case the SPD should be there, not at the CU. The important conductors in terms of the residual voltage drop are the ones through which the bulk of the surge current passes , not any others that happen to connect to high impedance victims. 
    Mike

  • Unless of course the surge is generated at the load end of things, in which case the SPD should be there, not at the CU.

    Is that (part of) the justification for having SPDs in the final boards? Obviously, it depends upon the nature of the installation, but you wouldn't want a surge sneaking back up a distribution circuit to the main board and then down another distribution circuit whilst the SPD is looking the other way.

  • This gets back to the question whether the SPD is best placed in the CU or in the isolator switch?

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