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back on the 'surge'

psychicwarrior

 Good morning to all.  Revisiting a surge issue please.

 

Imagine a final circuit (or it could be some equipment cabling e.g to radio mast but Im not familiar so prefer to use a final circuit as the example ) that is fed from a [upfront] SPD protected board . The circuit runs back outside  (across a zone boundary? ), overhead/buried, for some distance (e.g 100m or whatever)  to equipment that is in contact with earth.

 

  • is it true that circuit could introduce a ‘surge' back into the installation just as the DNO supply may at the origin ?
  • in such a case if true, would there need to be another SPD after the final circuit's OPD to cater for this **… or if not where would the SPD(s) go and across which conductors ?
  • what ‘surge’ levels coming back in to the installation might one expect ?

 

e.g. origin----SPD---DB/CU outdoor circuit OPD--- **additional SPD ------ overhead/buried run (100m) ---- [grounded?] outdoor equip (poss with local SPD)

 

Sorry in advance if I am not understanding well enough to ask a coherant question ! :-)

 

Thank you, Habs

  • 0

    The first question to ask, is whether the mast is high enough for BS EN 62305 to apply because of direct strikes?

    If yes, then definitely there is an issue and you need to adopt BS EN 62305.

    If not, then yes it's possible if you are going outside the particular protection zone … and the guidance is in Section 534 regarding zones.

  • 0

    In big handfuls, yes, actually almost any wiring has the potential to have currents induced in it in a way that causes volts to pile up in a bad way somewhere. The chances of those induced signals being large enough to cause problems, enough that they need to be clipped or filtered out in some way depends very much on what is being protected from what. There is no magic about the DNO wiring, except there is a lot more of it and you have less control of where it goes, so it makes a bigger ‘antenna’ and may also share with sources of hostile waveforms on other premises..

    So what raises or lowers the level of risk -  elevated wiring is more exposed than buried, and as Graham points out in the limit you could be providing the earth path for a lightning conductor.

    That is not the average garden light however, unless perhaps it is a floodlighting tower.

    Things with a larger thermal mass, transformers, motors etc do much better against surges then electronics, generally. So be less worried about an outside light spurred off the supply to a simple pump, than to one with a complex computer control.

    In your ascii- art the ** is indeed one place you may add additional suppression. But it can only clamp the voltage on LNE to be close to each other.  On its own it cannot prevent currents that flow in on one side of the building and out the other. LNE may be close to each other at the origin and also the exit of a large building, but the two locations may be kV apart for a few microseconds during an impulse event. Despite the odd journal article to the contrary a fatter CPC between the 2 locations does not help much, as there is not time for the electrons to get to the other end and back and realise they are connected.

    The problem is that the answer is very layout dependent, and there is a lot of advice out there that is only good in a limited set of circumstances.

    On a 230V system, most cases are on the edge of not mattering - which is why folk are only bothering about it now, though there is more vulnerable kit out there than a few decades ago, so the price of failure is slowly getting higher..

    Putting real numbers on a situation is very hard without an awful lot of information, most of which you never have.

    Mike.

     

  • 0

    Thanks Graham, Mike.

    I can appreciate that a comparatively short length eg 0.1km of single overhead/buried final circuit cable outside is perhaps not as much a risk, compared to the possible DNO 1km+ of ‘antenna’ delivering it's various nasties.   

    (mapj) “On its own it cannot prevent currents that flow in on one side of the building and out the other”

    That's the bit I think I'm curious about.  In the same way one is trying to arrest a current ‘coming in’ from the DNO supply side, what else is needed to do so for an induced current ’coming back in' from some outdoor lengthy installation circuit etc ?

     

    As an aside and briefly, various manufacturers seem to be suggesting that where there is an overhead DNO supply and regardless if there is an LPS or not employed, a Type 1 SPD should be installed  (poss a t1+t2 combined where appropriate). One has confirmed this to me directly.

     

    Regards

    Habs

     

  • 0

    Hmm. how to explain.

    All a traditional spd can do is to connect all 3 cores together at one point, so to the transient they behave as one.

    There is nothing stopping a voltage gradient along that one (three).

    Contrast with winding all 3 cores around a magnetic core  and exciting the core to make them the secondaries of a transformer - now all 3 wires add however many volts per turn* no of turns, but the difference between L and N is the same both before and after the addition.

    (if we add 100V onto L, N and E the differences stay the same - the SPD will not see any change and will not operate - but get something between the hot and cold versions of the E and it will not like it.)

    Mike.

  • 0

    @mapj1 thank you for that.  A little bit clearer it is becoming.

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