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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.

  • AJJewsbury said:
    I've been gradually retrofitting SPD to my own installation

    Gradually?

    I see that you have two (pairs of) SPD. Is that a cascade of type (1), 2, 3 or 2 x 2 on account of circuit length?

    Please ignore me if I have misunderstood the situation. FWIW, I chose to put SPD adjacent to my new house DB and since new populated CUs seem to include them, there is no longer much choice.

  • With the circuit isolated, the circuit's L and N will be floating (give or take a little bit of deliberate or incidental capacitive coupling). So I don't see why the circuit SPDs (the RH ones in your diagrams) would start to conduct.

  • The clue is, between 'Feet on 'Earth' and 'Earth' there is usually no potential difference... however, due to a surge, if the circuit is "long", possibly there may be an issue if the impedance through terra-firma, and/or the impulse current-wave through terra-firma, is sufficient ... but isn't that (most likely) less than the overall risk of being struck by lightning (or affected by a transferred lighting impulse or side strike) ?

  • 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.

  • Gradually?

    Yeah, I know, not ideal when you're trying to construct a consistent LPZ etc, but needs must. First the detached garage CU (on its own TT Earth) got some (as I was putting some new LED lights in there), now the house CU has got a pair. I'm still pondering what (if any) I should put around the PV system/inverter and TV aerial downlead (the phone line is likely to be replaced by fibre in the foreseeable, so that one less entry point to worry about).

       - Andy.

  • 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.

  • AJJewsbury said:

    3. During a surge..

    I'm struggling to see how touching the "isolated" live conductors in this situation is any more dangerous than touching cpc's or exposed-conductive-parts connected via the "isolated" circuits.

    Without the SPDs, there will be inductive and capacitive coupling of the impulse to the live conductors from cpc's (and in the "supply/distribution circuits" vice-versa)... so I'm not sure any danger is added.

  • in reply to gkenyon


    I'm not sure any danger is added
    I'm inclined to agree - the thing about that circuit that is 'missing' is the impedance to earth, so we can only estimate the voltage impressed onto the CPC and any cases of connected  appliances. Actually preventing the line conductors being left behind when the case pulses live may be better for the equipment.  
    The person holding the wire as shown, may just as well be holding the circuit CPC - generally there is far more of that exposed. Also realise that humans are much more robust to short duration shocks/over-voltage than equipment - the peak voltage on a well configured electric fence can be as much as 10,000, but it is a short pulse,   about a tenth of a millisecond (*); So while there is pain, there is no permanent ill effect. Lightning surges are of comparable duration.


    It probably is not a good idea to be rewiring such a system in a thunderstorm and you may feel the effect of the nearby strikes if are holding the wrong bits, but unless it makes you fall off a ladder, it's not an immediate danger - and not more of  a danger than the same situation without the SPD.

    Mike.

    For those interested in such fences this thesis has a good introduction to the current state of the art before describing his own.

  • in reply to gkenyon

    I'm struggling to see how touching the "isolated" live conductors in this situation is any more dangerous than touching cpc's or exposed-conductive-parts connected via the "isolated" circuits.

    In principle, agreed. In practice though we are a bit aware of the dangers of exporting PE - e.g. preferring class II equipment outdoors, so having those same dangers appear on conductors we've just gone to some effort to make safe, might take some unawares.


    Without the SPDs, there will be inductive and capacitive coupling of the impulse to the live conductors from cpc's (and in the "supply/distribution circuits" vice-versa)... so I'm not sure any danger is added.

    Presumably capacitive coupling etc poses a much higher higher impedance than an SPD (otherwise there's be no point installing SPDs) - so the magnitude of the available current and voltage would presumably be higher than without SPDs present (unless the surge was high enough to break down the normal insulation between conductors).

       - Andy.

  • in reply to AJJewsbury
    AJJewsbury said:
    so having those same dangers appear on conductors we've just gone to some effort to make safe, might take some unawares.

    But the comment is equally valid to a more likely situation, where you have an isolated circuit, which, because of connected equipment effectively means the isolated live conductors are effectively (fortuitously or otherwise) connected to the cpc, and a fault occurs somewhere else in the installation.

    Or ... you are unlucky enough (regardless of whether working on the installation at the time) to be in contact with exposed-conductive-parts or other conductive material connected to the earthing system of the installation somehow, and a fault occurs ...

    AJJewsbury said:
    Presumably capacitive coupling etc poses a much higher higher impedance than an SPD (otherwise there's be no point installing SPDs) - so the magnitude of the available current and voltage would presumably be higher than without SPDs present (unless the surge was high enough to break down the normal insulation between conductors).

    Because of the fast rising edge (lots of high frequency components) of the impulse, capacitive coupling presents less impedance ... but resistive coupling (conductors themselves) presents inductive coupling and greater impedance

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