110 V Across Earth Terminal's...any Ideas

[Adam Raynor]

Hi

I'm wondering if any one can help with a problem I'm having.

We've installed a mccb panel which supplies 4 x 3 phase boards. The circuits test ok with regards to insulation resistance, but when I disconnect the earth from the earth terminal with the MCB on I can measure 110v between the disconnected earth cable and the earth bar in the board.

Any idea's?

Also the earth is connected to the neutral at the suppliers incomer

 

[NozSpark]

Why are you disconnecting the earth while you have the circuit energised?

That is a very bad idea...

 

[Adam Raynor]

Not really the point, it was noticed during testing.

Can you contribute anything useful?

 

[Sidewinder]

What are you measuring the voltage with (exactly)?

What is the supply type?

How far from the origin is this part of the install?

What is the general arrangement in the area?

e.g. is it a panel board, a single MCCB in an enclosure?

What sort of current is flowing in the circuit, and, if there are others around, in those also?

What is the lighting in the area?

What is the wiring type and installation method for the "suspect" circuit?

 

[Adam Raynor]

The voltage is measured with a set of meggar test probes

Type is TNCS

Distance from origin is 5 and 39 metres a few board's

No idea what current flow is, lighting circuit on 10a MCB

Any earth removed from the various board shows a voltage between the disconnected earth cable for that circuit and the earth bar in the board

No idea of general arrangements in the area

Type of wiring is FP 200 or the latest equal,which run on tray

 

[apprentice87]

Measure the current then, a few millamps i expect, does it spark when you pull the earth out??

john..

What is the impedance of your test meter. [the test probes]

 

[NozSpark]

Ok, so

your earth wire is disconnected from the earthing arrangement and is running parallel with some LIVE cables?

 

[apprentice87]

Ah!!!! Good thinking!!!

john....

 

[Adam Raynor]

Ok, so

your earth wire is disconnected from the earthing arrangement and is running parallel with some LIVE cables?

Yes

 

[MajorOak]

Induced voltage from the line cables.  If they are not tied down to earth, then the earth cables are floating and will experience electromagnetic induction from the lighting cables.

 

[Adam Raynor]

Induced voltage from the line cables.  If they are not tied down to earth, then the earth cables are floating and will experience electromagnetic induction from the lighting cables.

I would expect some inducted emf, but 110 seems and bit high and the lighting cables have a metallic sleeve to prevent this. I've never experienced more than 15v

 

[MajorOak]

Do the lights have any kind of electronic ballasts or power supplies?  Not sure what you are powering up but to help with EMF issues, capacitors and inductors are used to filter noise on SMPS and the caps are connected to the shielding / gnd.

If the earth is not connected the voltage on the earth path can shoot up to line voltage, 110v is 1/2 L-N so potentially (excuse the pun) 110v indicates some sort of centre tapped transformer use?

 

[NozSpark]

It might show as 110V with a high impedance DVM,, wrong tool for the job

 

[MajorOak]

yeah, true it could be up at any voltage if it's not tied down anything, it's basically a simple current transformer.  You have current flowing in the primary and an open open circuit secondary.

 

[Sidewinder]

The voltage is measured with a set of meggar test probes

...

Ok so you are using Meggar {sic} test probes.

What meter are you using, and what is the input impedance of said meter?

Are you using a Megger, multi-function tester, perhaps a 15xx or 17xx series?

Or, are you using a Megger digital multi-meter (DMM)?

 

[Evans Electric]

I'd go with induced voltage too , FP200 eh.!      You'll get it with X grades in trunking & Twin/Earths if you pull the earth connection  .

Just as an experiment try a 110 V  lamp across your floating earth & the  earth bar .

And on office power sockets , PCs use the earth  wire to "dump" Data which has been known to trip RCDs  I'm told.

May I ask the reason for using FP200 .

 

[apprentice87]

Hi All,

110V to earth from the earth bar... Normal leakage to earth from a load???

A 400V supply is 230 phase to earth. In this case R1 [from the Tx] and R2 [back to the Tx] are about equal, and therefore, as they are measuring from the "central point" for want of a better term, they have made themselves a "voltage divider" and therefore they are measuring half the normal voltage from phase to earth??????

Just a guess, might be miles out, but there you are..

john..

 

[binky]

think he may mean voltage probes?

when i first started out and had a bad habit of sticking test meter on all sorts of things whilst changing simple boards i used to get all sorts or spurious readings like the one described, and worry like hell about it all. Soon learned not to be so nosey and stick to the presribed tests only.

My vote is simple induced voltage in cpc and they can certainly get quite high.

 

[Sidewinder]

think he may mean voltage probes?

when i first started out and had a bad habit of sticking test meter on all sorts of things whilst changing simple boards i used to get all sorts or spurious readings like the one described, and worry like hell about it all. Soon learned not to be so nosey and stick to the presribed tests only.

My vote is simple induced voltage in cpc and they can certainly get quite high.

That may, have something to do with my questioning WHAT the measurements were made with.

A high impedance DMM/MFT with an input impedance of Mega Ohms per volt will measure "phantom" voltages which cannot actually even be detected by the Human body.

If he were using say an Avo 8 & getting 110V then I would suspect an issue.

I bet that the OP does not even know which "way" the potential "is", from the outside in, or from the inside out as it were.

That is, there ain't a screwdriver hole in the lawn yet IYKWIM.

 

[binky]

screwdriver earth rod? Never thought of that one :^O

if you poke around any electrical board you can get all sorts of strange readings. The only thing I do like doing as a result of confusing the hell out of myself years ago, when changing a board, is using the volt probes to find linked circuits, turning off all the MCBs and checking top  terminals on all the other 'dead' mcbs. It's a quick and easy thing to do.

Haven't seen an AVO 8 for many a year. I still like anologue test gear, digital is nice, but just not the same. Software just doesn't function the same as good old fashioned basic principles, though i would have to revise what they are now :slap