Induced Voltage LED / Steel Trunking

[davesparks]

Just seemed very high for phantom voltage.

The phantom voltage could be anything up to the highest potential present in the trunking.

 

[The Ghost]

@Sidecar Man as a matter of interest what did you get 38v between. To put it another way you put one pole of your voltage tester on the "dead" cable and the other pole on??? Is the cable connected to anything at the other end?

 

[bomjac]

Simply you have induced voltage from some of the cables in the conduit. The current in these cables induces a voltage to in those cables which are not connected.

When you connect a DMM across the spare cables and ground the impedance of the DMM (10Mohm or more !) is so high that it sees this induced voltage.

My back to back (MSc qualified) spent hours offshore chasing this phantom voltage on an MCC before he sent me an email in the office and I gave him the reasons.

For this same reason the Dutch regulations (NEN) only allow testing for dead with a dipole tester. This tester has a button which when pressed increases the current flow through the tester to 100 mA.

This is why I prefer to use the old square D type coil based voltage tester over a DMM for proving dead.

 

[Deleted account]

Bin your volt stick. Get an aproved voltage indicator.

 

[Lucien Nunes]

As per Davesparks the voltage you read could be anywhere between zero and 230 depending on the potentials of the connected cables that the floating cable is lying next to. The actual voltage is pretty meaningless because it's not part of a recognisable circuit. Testers with different impedances will give you different numbers, typically it will be somewhere between zero and 115.

The current that you can draw from it will be microamps which is why even a high voltage is unimportant. You can charge a balloon up to thousands of volts by rubbing it against your jumper, but the charge stored is so low you won't get a useful measurement with any normal instrument. Like the balloon, the floating cable is just a conductor with some stray capacitance to things nearby.

 

[Chris1234]

38 volts? It could be connected to the Siemens 24v PSU that I removed from a machine last week!