weird lighting fault

[Dean Williams]

Hi folks
while testing this theatre, i attempted to get a Zs from an emergency exit sign. The steel conduit is being used as cpc.
when i connect my three probes to l,n,earth i'm getting 110v on the screen and when i press test it comes up with about 160 ohms.
i put a voltmeter across and found 110v between L and E but 230v between L and N.
could this be a poor connection on the steel conduit cpc and if so, why does the voltage drop to approx 110v

thanks...its a bit of a puzzle

 

[telectrix]

do a R2 test on the conduit end to end. then test to MET.

 

[Dean Williams]

from the board to the fitting is about 300 metres...

 

[telectrix]

even if it's 3000m you've got to get it right, one way or another. do it in stages with a wander lead.

 

[Dean Williams]

that will be fun, all the conduit is buried in walls and in ceilings to which there is no access. Thanks

 

[Dean Williams]

assuming it is a bad earth, can someone please explain the science behind it...how does it become 110v???

 

[Guest 004]

sounds like a break in the conduit to the fitting, just code 1 it, and recommend rcbo

 

[telectrix]

code 1 it and FIX it. you don't put a band- aid on an arterial bleed.

 

[Dean Williams]

thanks. the conduit is 35 years old!
just dont understand why its 110v

 

[Dean Williams]

i will....i just want to get my head round the reason as to why the voltage drops to 110v

 

[Guest 004]

probably getting a reading through the damp walls, and forgot to mention, you should not walk away from a code 1, without isolating or rectifying.

 

[telectrix]

floating earth?

 

[Dean Williams]

i'd be there forever and a day if i rectified all 76 code 1s that ive found so far!!!

 

[Guest 004]

There should be an agreement of what to do, as you find the code 1's

 

[telectrix]

only 76, must have been done on a good day.

 

[mogga]

FFS I realy hope you have a contract saying code 1s to be repaired

 

[platty1005]

I assume you are doing a PIR ?
Conduit as CPC 35 years old
76 code 1's so far I guess the recommendation is a full re-wire
As for 110V L-E try testing much closer to the main DB
I guess the conduit is badly corroded at various points and is a very high resistance compared to L-N insulated copper conductors

 

[WDMDL]

assuming it is a bad earth, can someone please explain the science behind it...how does it become 110v???

If you've got good Zs figures elsewhere you can rule out the earthing conductor itself.
So if we're dealing with a fault on just this one conduit run, here goes:

Let's assume:
The conduit is completely open circuit quite near the origin.
The insulation resistance L-E is 0.5Meg and N-E is 0.5 Meg. (measured at end of circuit)
Voltmeter input impedance is 10Meg.

Think of a potential divider, L to N, with the conduit in the middle
For a 230V supply, you'd get a theoretical 115V
But with the 10 Meg of your voltmeter in parallel with whichever 0.5Meg you were measuring across, the voltage seen would reduce slightly.

Now if you've got a measured Zs of 160 Ohm and 110V L-E, sounds like something is corroded AND full of water

Unless someone's made up an IT system, centre tap earthed through an impedance....

Simon.

 

[Knobhead]

Try using an old analogue meter and then check the voltage. I hate DVM’s give me an AVO any time!

 

[Dean Williams]

Thanks for the explanation...i understand it more now.

I'll try it with an analogue voltmeter tomorrow. Thanks