Earth cores in AC and DC cables
Earth cores in AC and DC cables
(OP)
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.
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.






RE: Earth cores in AC and DC cables
Regards
Marmite
RE: Earth cores in AC and DC cables
Bill
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"Why not the best?"
Jimmy Carter
RE: Earth cores in AC and DC cables
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.
RE: Earth cores in AC and DC cables
RE: Earth cores in AC and DC cables
Regards
Marmite
RE: Earth cores in AC and DC cables
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?