Getting 10v on Steel Capping

[DPG]

Capacitive coupling from the cable to the capping?

 

[Avo Mk8]

Capacitive coupling from the cable to the capping?

Agree that's a possibility, especially if measurement being made with a multimeter?

 

[Lucien Nunes]

You said:

I got a reading of 10v.

Without specifying which two points the 10V was measured between. The capping and ?
Earth, we might presume but it should always be stated.

Did you expect there to be no voltage between the capping and earth? Is the capping connected to earth? If not, why would you assume them to be at the same potential? If the capping is not connected to anything, it is free to take up any potential it likes, 10V, 100V, 1000V to any other thing. But there is a small amount of capacitive coupling from it to everything around it and small currents can pass through the capacitive reactance to induce or remove a potential on the capping. It's the same principle that a Volt-Stick uses to sense voltage without a connection.

Most of the stray capacitance from the capping is to earthy things around it, so it will tend to take up an earthy potential. But if there are energised line conductors nearby, e.g. the enclosed T+E, there will be a small amount of capacitance to line. This works like a voltage divider and will pull the capping to a voltage between the two but much nearer earth than line. The actual voltage tells you little, other than that the capping is near some live cables. It's often called a 'ghost voltage' - it's still a real voltage but its source has such a high impedance that it is hard even to measure consistently.

Q: How would you go about distinguishing between:
A) 10V ghost voltage relative to earth, on a piece of capping that is genuinely floating, i.e. not in contact with any conductors, and
B) 10V relative to earth, on the same piece of capping that has come into solid contact with a conductor 10V from earth, e.g. where a fixing has hit a neutral conductor.

 

[telectrix]

A) 10V ghost voltage relative to earth, on a piece of capping that is genuinely floating, i.e. not in contact with any conductors, and
B) 10V relative to earth, on the same piece of capping that has come into solid contact with a conductor 10V from earth, e.g. where a fixing has hit a neutral conductor

I'd start by using a "proper" meter:

 

[timhoward]

A) 10V ghost voltage relative to earth, on a piece of capping that is genuinely floating, i.e. not in contact with any conductors, and
B) 10V relative to earth, on the same piece of capping that has come into solid contact with a conductor 10V from earth, e.g. where a fixing has hit a neutral conductor

I'd start by using a "proper" meter:

Ooh...drool....you tart! You'll be saying you have one for it next:

Q: How would you go about distinguishing between:
A) 10V ghost voltage relative to earth, on a piece of capping that is genuinely floating, i.e. not in contact with any conductors, and
B) 10V relative to earth, on the same piece of capping that has come into solid contact with a conductor 10V from earth, e.g. where a fixing has hit a neutral conductor.

Seeing as no one has answered, I think easiest is to disprove B with an IR test. Once B is eliminated it becomes A by default.

 

[LastManOnline]

Q: How would you go about distinguishing between:
A) 10V ghost voltage relative to earth, on a piece of capping that is genuinely floating, i.e. not in contact with any conductors, and
B) 10V relative to earth, on the same piece of capping that has come into solid contact with a conductor 10V from earth, e.g. where a fixing has hit a neutral conductor.

10V relative to earth would produce some arcing when a wire is connected between the capping and earth.

When the same procedure is carried out with a 10V ghost voltage no arcing should occur

 

[Lucien Nunes]

I really aimed that question at the OP who is a trainee. Good to see some old hardware still in use though.

 

[7029 dave]

Still got my 500V analogue IR meter, came out last year for some fault finding.

 

[davesparks]

10V relative to earth would produce some arcing when a wire is connected between the capping and earth.

When the same procedure is carried out with a 10V ghost voltage no arcing should occur

I'd be interested to see the RAMS for that test procedure!

 

[telectrix]

I'd be interested to see the RAMS for that test procedure!