Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • Replies 11 replies
  • Subscribers 36 subscribers
  • Views 1007 views
  • Users 0 members are here
Options
You may also be interested in

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.

Parents

  • 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.

Reply

  • 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.

Children
  • 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

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines