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# tn-c-s help

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B

#### b0nes

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

hi all.

i may be completely wrong but on a tn-c-s system if i was testing at say a lighting point, lighting circuit breaker off but main switch on then i would have continuity between neutral and cpc, right? but then by my reckoning if i were to then open the main switch then the continuity would be broken due to the neutral going open circuit, also right? its just i have read that IR readings could be measured wrong if the neutral for the given circuit is not removed from the neutral bar when testing IR, is this true? and why would this be a problem if the main switch was open?? sorry am just learning and a bit confused....any help/thoughts would be great

T

#### TonyM58

Bones,

Hhmmmm, bit of a jumble there.

Ok in all three standard systems,(TNS TNCS & TT) the neutral is connected to earth at the star point of the distribution transformer. So the neutral will allways have continuity to earth at different readings (with TT its got to go back through the ground)

Now course, the neutral is a live conductor, so i would hope you were not measuring between neutral and earth on low resistance ohms???!!!

Its confusing because you are talking about continuity and then IR

You would not remove the neutral when IR testing because you would either be testing :

at the tails beofre energising, with the main switch on, in case the N and Earth would not be going anywhere (and would not be connected)

OR

On the dead side of the open isolator, (with power coming in) in which case, as you correctly say, the neutral to earth path would be broken anyway

So,... theoretically, yes if the main switch was closed then there would be little or no insulation between neutral and earth, but i cannot concieve of any situation where:

you could measure it

OR

you would want to!!!!

hope i have understood your question, and hope this has helpled

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