AC Single Core Armour Grounding 1

[zaza123]

We are working on AC Single core armoured cable, system voltage 34.5 KV. The armour earthing shall be done on both sides or single side?. I have read that connecting the armour to earth on both the sides will lead to flow of circulatng current & will increase the heat in the cables. The cable length in our case is 450 meters.
Also if single side which side is poreferrable supply point or source point.

Thanks in advance.

 

[WhiteyWhitey]

Zaza123,

Search the forums for keywords Cross Bonding, Sheath current, etc. There are quite a few posts on this topic.

Earthing the armour at both ends will induce a circulating current in the armour and the related losses and derating of the cable.

Earthing at one end will eliminate the currents but induce a voltage at the un earthed end. If this voltage is calculated to be acceptable then single point earthing is acceptable.

Thanks,
Andrew

 

[alehman]

Unless you are concerned about that one bond coming loose. After that happens, the sheath will be at line voltage until it shorts to something else.

Alan
----
"It’s always fun to do the impossible." - Walt Disney

 

[zaza123]

There is a recommendation from the Contractor that the Sending end armour Shall be earthed. We have same voltage at the Sending end & the recieveing End ( 34.5 KV). At the recieving end there is a CT for the protection of the cable through Line differential relay.If we earth the Sending end only what about the armour passing through the CT. Does the armour there need to be earthed also. If not then can the circulating current in the armour trip the differential relay.

 

[Marmite]

If you go for single point bonding then you will need to use insulated island glands or something similar at the unearthed end. The sheath standing voltage will need to be calculated to ensure it is not a risk to personnel. The sheath voltage will rise in the event of transients and other faults. You may need to install sheath voltage limiters, also an earth continuity conductor end to end. Use of single point bonding is not a decision to be taken lightly, it has to be done properly, and backed up with calculations. I would earth the cable at the end most frequented by staff, eg the switchgear end rather than the transformer. Having said all that, personally in this case at 34kV I would use solid bonding and lay the cables in tight trefoil, unless there is a very high current and spacing issue.
Regards
Marmite