Earth cores in AC and DC cables

[veritas]

I'm currently doing a job for a customer where by default every earth core in a cable, whether AC or DC, is earthed at both ends. I raised my eyebrows at this mainly because of my concern for the adverse effects of induction on the other cores in the cable under earthfault conditions.

In particular,

1) I have never seen earthcores in CT cables being earthed at both ends. Usuaally only the 4 CT cores are used with all spare cores (if any) safely terminated but not connected to anything. The neutral core of the CT is earthed in the protection panel. Does anyone earth the earth core in a CT cable both ends?

2)In DC signalling cables (such as those out to the circuit breaker) I have always seen the screen earthed where applicable but not the earth core. Again my concern is for inductive coupling between the earthcore and the DC cores in the cable. Should there be a distinction made between treatment of the earthcore for a 50V DC and a 110V DC cable? I think not but thought it good to put the question out there.

3) The only time I see the earthcore being used is in an AC LV power cable where it acts as a protective earth and is connected to the MEN.


Thanks.

 

[Marmite]

CT secondary circuits should be earthed at one point only, otherwise you will get circulating currents which may affect the stability of the protection, particularly during earth faults. The protection could trip for a fault on a completely unrelated circuit.
Regards
Marmite

 

[waross]

I remember a glowing, smoking, red cavity in an MCC that was a high end PLC before the fault current from a workers extension cord inadvertently returned on the earth core of an I/O cable.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[veritas]

marmite - I am familiar with the practice of only earthing the CT neutral at one end. The issue I am faced with here is a different one. There is a seperate earth core in the CT cable, thus nominally 4 CT cores + earth core (like a seperate wire inside the CT cable). This was earthed at both ends.

I have asked for this to be removed as circulating currents again are my concern. Same as if the CT neutral is earthed both ends.

I am interested to know what do other companies do?

The question is extended to signal cables such as those between the control room and a ciruit breaker. Is an earth core a requirement when a cable has DC > 50V?

Thanks.

 

[dpc]

Many utilities in the US use shielded control cables in substation at 230 kV and higher. It's been a long time since I've dealt with this, but I seem to recall that these shields were grounded at both ends. There may be some circulating current, but the circuit length is fairly short, so I would not expect it to be troublesome in a CT circuit, but our CT secondary current is 5 A, not 1 A.

 

[Marmite]

I'm wondering whether this is a local wiring code issue where the cable armouring isn't of sufficient cross section on its own to act as a protective conductor under the code. Perhaps the CT cables are doubled up to reduce the burden which has increased the copper equivalent area required in the armour?
Regards
Marmite

 

[veritas]

Marmite - No, there is just a blanket practice to have an earth core in ALL cables (whether AC or DC) and to earth this earth core both ends. Nothing to do with actual application (burden, fault rating, etc). I have indicated possible induction problems with this earth core in a CT cable. But I am also concerned with this earth core being used in DC signalling cables (affectively AC in a DC cable under EF conditions). In LV Power cables, yes, I can understand its need as a protective earth.

I think it is pretty clear that it is not a good idea for CT circuits.

dpc - interesting one. Can current in a screen induce voltage in the cores inside the cable? I would imagine that the magnetic field radiates out from the screen and no B inside the cable?