What decides whether an SPD actually protects anything

1. The number that matters is not on the datasheet

An SPD's voltage protection level is measured at its own terminals. The equipment is not at the SPD's terminals — it is at the end of some wiring, behind some connecting leads. What has to be compared against the equipment's rated impulse withstand voltage is the effective protection level: the device's level, plus everything the installation adds on top of it.

That distinction is the whole of this article. A device with an excellent protection level, installed with long leads at the wrong end of a long circuit, can deliver a worse result to the equipment than a mediocre device installed properly. The datasheet is necessary and nowhere near sufficient.

2. The half-metre rule, and what breaking it costs

The standard limits the total length of the SPD's connecting leads to half a metre. Total is the word that gets missed: it is the conductor from the live line to the SPD plus the conductor from the SPD to the earthing terminal, not just whichever one is longer or whichever one you can see in the photograph.

The reason is inductance. Surge current changes fast, and a fast current change through even a short conductor develops a real voltage across it. That voltage is in series with the SPD, so the equipment sees the sum. As a scale: at ten kiloamps through the device, one metre of lead develops roughly a kilovolt on the standard's own approximation — comparable with the entire protection level of a decent low-voltage device, thrown away by routing.

Two honest qualifications, because this is where a tool can quietly flatter you. The standard's formula is explicitly an approximation, and it says the true value depends on the current waveform, the conductor arrangement and the loop geometry. Published manufacturer figures for the same case run higher than it — one major vendor's catalogue gives about 1.2 kV where the formula gives 1.0 kV for that one-metre, ten-kiloamp case. We compute the standard's figure and say on the report that the real one may be up to about a fifth higher, because the alternative is presenting a number we know to be optimistic without saying so.

That gap is exactly why half a metre is a requirement rather than advice: kept short, the disagreement between the formula and reality does not matter. Let the leads grow and it does. Where the half metre genuinely cannot be achieved, there are three recognised ways out — choose a device with a lower protection level, add a second coordinated SPD close to the equipment, or use the standard's V-shaped connection arrangement so the lead voltage is not in series with the protection. Accepting the long leads is not on the list.

3. Ten metres, and why distance is not just attenuation

Intuition says a longer run between the SPD and the equipment can only help — more impedance, more damping. Intuition is wrong here, and the error is in the direction that hurts.

A surge travelling down a circuit reflects at the far end, where the impedance changes. The reflected wave adds to the incident one, and at an open or high-impedance termination the voltage at the equipment can approach twice the value the SPD is holding at its own terminals. The standard handles this with a reflection factor between one and two, and requires the worst case of two to be assumed unless a detailed analysis says otherwise.

So the rule splits at ten metres:

• Equipment within 10 m of the entrance SPD. The effective protection level should not exceed 80 % of the equipment's rated impulse withstand voltage. The 20 % is the margin absorbing what the calculation does not model.
• Equipment beyond 10 m, protected only by a one-port SPD at the origin. The available withstand is effectively halved, which is the reflection worst case made explicit. Many installations that look compliant on the datasheet fail here.
• Equipment beyond 10 m, done properly. Either a second coordinated SPD close to the equipment, or a two-port SPD at the origin. Both restore the full withstand — but the origin-mounted route carries a condition that is easy to skip: it requires shielded wiring throughout the protected circuit. Claim the two-port allowance on unshielded wiring and the calculation no longer applies.

The practical consequence for a real building: a single entrance SPD does not protect a distribution board two floors up, however good it is. That is a coordination problem, and it is solved with a second device, not a better first one.

4. Zone boundaries decide how many devices you need

The lightning protection zone concept is the part of Part 4 most often reduced to a diagram in a tender document and then ignored. It is doing real work: it is what turns "put in a surge protector" into a number of devices in specific places.

The rule is that an SPD belongs at the entrance of a line into each inner zone — and where only one protected zone exists, at least one at its entrance. The zones themselves are defined by what a location is exposed to: whether a direct strike can reach it, whether the full lightning electromagnetic field is present, and how much surge current can arrive. A boundary is where one of those things changes, and every crossing is a place where the conducted surge has to be dealt with.

There are two recognised ways to reduce the count, and both are design decisions rather than omissions: interconnecting zones of the same order so they behave as one, and extending a zone into the next using shielded cable bonded at both ends, so the run itself carries the protection. Both are legitimate; neither is "we only fitted one".

This is also why an SPD schedule is a design output rather than a shopping list. The count, the locations, the class at each location and the coordination between them all follow from the zone layout — which is why our tool produces a manufacturer-neutral schedule you can issue to a supplier, rather than a single recommended part number.

5. How to check any of this, including ours

Three questions to put to any surge protection design — one you have been handed, one you have produced, or one a tool has produced for you:

1. Does the comparison use the effective level or the datasheet level? If nothing in the calculation accounts for the connecting leads, it is comparing the wrong number against the equipment withstand.
2. Is the distance to the protected equipment an input at all? A design that never asked cannot have applied the 10 m rule, which means it silently assumed the favourable case.
3. Does the device count follow from the zone boundaries? If the answer is one SPD because there is one incoming service, the zone concept was skipped.

Our own working is public, including where it disagrees with other published sources — the lead-voltage figure above is one such case, and there is an unresolved question about how one manufacturer's handbook reads a table in the series that we set out rather than quietly pick a side on. It is all on the validation page, with the sources named. If you run these checks against our output and something looks wrong, that is the message we most want to receive.