85v live to earth

[E.E.I]

Evening all,

i was changing some down lights today and when I came to live test the lights I noticed I was only getting 85v live to earth? Also neutral to earth is showing 50v and live to neutral 230v.

im guessing it's a neutral fault somewhere, I will be back at the job tomorrow to investigate further. Just wondering if any one has had a fault like this before and what the cause was?

Thanks

 

[HandySparks]

Evening all,

i was changing some down lights today and when I came to live test the lights I noticed I was only getting 85v live to earth? Also neutral to earth is showing 50v and live to neutral 230v.

im guessing it's a neutral fault somewhere, I will be back at the job tomorrow to investigate further. Just wondering if any one has had a fault like this before and what the cause was?

Thanks

What prompted you to measure the voltage? This isn't one of the usual live tests.

Under what conditions were these voltages measured? (On load / off load?)

What were your dead test readings?

What were your other live test readings? Zs?

 

[Octopus]

my money is on no CPC continuity!

 

[E.E.I]

So the reason I spotted the issue was my Meggar shows the voltage before carrying out the zs test.

i only got as far as Ze at the db 0.37

im going back tomorrow to investigate tomorrow

will start with Continuity of cpc

 

[telectrix]

agree. floating earth.

 

[SJD]

As mentioned, almost always this turns out to be a CPC that is not connected somewhere, it floats to somewhere between 230 and 0 depending on the loading (e.g. your test meter). Assuming the old light(s) was/were working before any work (it always pays to check!) it is unlikely to be a N fault.

Could well be at another light fitting, but could also be somewhere else, e.g. a junction box, damaged cable, etc. Often found after previous DIY work, or a builder that prefers to cut off CPCs on the lighting cables rather than connect them.

 

[Octopus]

Or a house with twin at the CU and twin and earth "later" in the circuit....

 

[E.E.I]

Well hopefully it's not a hidden JB and just a loose connection at a light fitting somewhere else in the house, luckily it's just centre lights in the rest of the house, nothing worse then pulling down down lights to find the last one you pull down is the one you need.

the property was tested 10 years ago, not sure if they have had any other work since.


i will update tomorrow when the fault has been resolved.

 

[gazdkw82]

My tester does the same.

mine had 1v on the earth yesterday

 

[E.E.I]

Just an update

first light I went to the cpc fell out of the terminal block. Job done :cheesy:

 

[hightower]

Could somebody explain this floating earth effect for me?

 

[SJD]

Could somebody explain this floating earth effect for me?

With twin & earth cable, if the CPC is not earthed, then assuming a good piece of cable, there is the capacitance from L to CPC, and N to CPC which are approximately equal. So if the cable is being used for L and N, the CPC floats up to somewhere around half the voltage on L.

Being very small capacitances, you can't get much current out of this CPC, even trying to measure the voltage will cause it to drop.

 

[gazdkw82]

So basically, if the cpc isn't terminated to earth and effectively just floating between L and N, the voltage within the L and N will induce a voltage on the cpc cable?

 

[telectrix]

think of it as a voltage gradient. you have a L conductor sitting at 240V and a N conductor sitting at 0V. sandwiched between these is a a conductor that is not tied down, so it floats somewhere between the two.

 

[gazdkw82]

think of it as a voltage gradient. you have a L conductor sitting at 240V and a N conductor sitting at 0V. sandwiched between these is a a conductor that is not tied down, so it floats somewhere between the two.

So if you come across the situation as OP, this is almost certainly a floating earth?