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Thought for the day ... Can SPDs undermine safe isolation?

AJJewsbury

I've been gradually retrofitting SPD to my own installation so have been pondering a few of the trickier cases and where the surge currents/voltages are likely to flow, which led me to an odd thought... If I have an SPD after a point of isolation (which is the usual arrangement in CUs etc) - is there a risk of the SPD transferring a nasty voltage back onto the (live) conductors that are meant to be isolated?

On the basis that SPDs work by creating a momentary (or short duration) short circuit - they'll raise the potential on PE as much as they reduce it on the live conductors (at least on a TN system) - that's fine of course as equipment will mostly only see the voltage difference between live conductors and PE. On TT I presume PE will be dragged up a lot more than L is dragged down ... but the p.d. between them is almost eliminated just the same.

My thought process went something like this...

1. Simple (say TT) installation, local isolation for a bit that's being worked on (perhaps outdoors?)

2. Add some SPDs..

 

normally still safe to work on..

3. During a surge..

maybe not quite so good.

Yes, some of the current will be diverted to terra firma by the installation's electrode, but likely not all of it.

The surge should be a very short duration (if from lightning or some such), but might be longer (e.g. if the overvoltage is due to a broken supply N or some other cause).

(And it does tie in with the old conundrum about isolating the N/PE if working outside the equipotential zone.)

Anyhow I thought I'd throw the idea out there to see what people thought. Mostly I suspect it's a non-issue, but maybe there's a few corner cases where the normal advise for safe isolation might be augmented with "..and pull out the SPD cartridges".

   - Andy.

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  • Maybe i am missing something. But in the tt scenario isn't the biggest risk all of the exposed conductive parts with large surface areas having a large voltage on them. Given its not going through the second spd in the diagram it will be about 273v above the voltage on the line conductor connected to the stick man.

    Would the best advice be not to work on an installation during a lightening storm. Not sure what you would do I  an industrial environment with large motors, maybe ensuring there is a very good earth is good enough.

  • in reply to Alan B

    Maybe i am missing something. But in the tt scenario isn't the biggest risk all of the exposed conductive parts with large surface areas having a large voltage on them.

    I think you have a point there - perhaps a risk that SPDs in general are increasing? Within the equipotential zone I'd hope that the risks were minimised, but the equipotential zone never was perfect and usually has edges, where things get a lot less comfortable.

       - Andy.

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  • in reply to Alan B

    Maybe i am missing something. But in the tt scenario isn't the biggest risk all of the exposed conductive parts with large surface areas having a large voltage on them.

    I think you have a point there - perhaps a risk that SPDs in general are increasing? Within the equipotential zone I'd hope that the risks were minimised, but the equipotential zone never was perfect and usually has edges, where things get a lot less comfortable.

       - Andy.

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