induced voltage

[adslev]

How do I get rid of them!
Testing a job another contractor has been working on. In the main hall of the building there is a switch bank of 7 switches and 7 emergency key switches. Each of the switches are on a separate circuit. When I remove one of the lighting circuits from the rcbo I get 80v on the live conductor, the only way to get rid of the voltage is to turn off all other rcbo's which are heading to the switch bank.
All circuits are on the same phase and at the switch bank there are
: 7 permanent feeds
: 14 strappers
: 7 emergency feeds
: 2 earth's
So basically 30 cables running to the switch bank down 3x25mm steel conduit drops and along 2x2 metal trunking.
Anyone experience anything similar and have any suggestions as to how best remedy the problem

 

[Silly Sausage]

It is just a 'ghost or stray' voltage, there is no energy in it. Its value depends on the instrument you are testing with.
Try measuring this disconnected 'Live' with a Low Impedance voltmeter and you should(!) read zero.

 

[adslev]

Tested with a megger 1730
The issue came about when I was about to carry out an insulation resistance test. The test couldn't be carried out as the test leads where picking up a voltage. Once all rcbo's where switched off each circuit tested clear > 999
Had heard of induced voltage was the problem and better segregation was required which would not have been an easy task as the Hall is finished and has quite long conduit drops.
Cheers and thanks for the speedy reply

 

[C&H]

Tested with a megger 1730
The issue came about when I was about to carry out an insulation resistance test. The test couldn't be carried out as the test leads where picking up a voltage. Once all rcbo's where switched off each circuit tested clear > 999
Had heard of induced voltage was the problem and better segregation was required which would not have been an easy task as the Hall is finished and has quite long conduit drops.
Cheers and thanks for the speedy reply

hard to say without seeing it but i'd guess theres a stray nuetral on a lighting circuit feeding back through the disconnected live

 

[adslev]

hard to say without seeing it but i'd guess theres a stray nuetral on a lighting circuit feeding back through the disconnected live

Don't think it's the case as the neutral runs into a klik box and there are seven klik boxes, one for each circuit.

 

[Engineer54]

Always a good idea to carry an analogue multi-metre in your test kit, these will almost always confirm (or not) if there is stray voltage or phantom voltage present!!

 

[PEG]

God bless the old kit,these new fangled meters will give you 230 through a damp dog hair...

 

[Silly Sausage]

Why don't they make all these MFTs with Low Impedance anyway?
For what they are used for 99.99% of the time.
Must check my Fluke's spec!

 

[bomjac]

This is the reason that Dutch regulations state you must use a "dipole voltage tester" to prove dead. These testers have a button that will introduce a 100mA drain current through the tester to earth, which should do away with the induced voltages. Problem with a fluke / multimeter is that the input impedance is in the order of Megohms, and can give falsely hight readings where high impedance earth faults exist.

Give me a dedicated voltage tester over a fluke any day for testing for dead.

 

[Engineer54]

This is the reason that Dutch regulations state you must use a "dipole voltage tester" to prove dead. These testers have a button that will introduce a 100mA drain current through the tester to earth, which should do away with the induced voltages. Problem with a fluke / multimeter is that the input impedance is in the order of Megohms, and can give falsely hight readings where high impedance earth faults exist.

Give me a dedicated voltage tester over a fluke any day for testing for dead.

A cheap analogue multimeter will do the job just as well. lol!!
But i'm fully behind you, on electricians having a dedicated voltage tester in their test kit.

 

[bomjac]

A cheap analogue multimeter will do the job just as well. lol!!
But i'm fully behind you, on electricians having a dedicated voltage tester in their test kit.

It will, but I wouldn't go anwhere near live busbar or heavy power terminal with anything other than a class III or class IV piece of test equipment.

 

[Engineer54]

It's a case of having the right tool for the job in hand. ....It's always worked for me!!

 

[Heisenberg]

This is worth a read...

Technical Resource Library from Cole-Parmer

Sorry about the US terminology.

 

[adslev]

Sounds about right, wouldn't mind one of them fluke stray voltage adaptor test lead sets but I doubt it if it'll work with the megger, have been looking into getting a multimeter but I can't bring myself to buy one because I was always told they were for mechanics and such, when I was doing the 2391 a few years back I remember the tutor saying that if he had an inkling that any of us used a multimeter he would fail us on the spot!