Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 7 replies
  • Subscribers 31 subscribers
  • Views 1241 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

Single Point or Solid Bonding query on 20kv U/G supplies

seansasta
Hi Guys

I'm reviewing a MV cable installation design proposal and I'd appreciate some advice, scenario as below...

New Building with new MV switchgear being fed from an existing Main MV switchroom within the same facility campus.

MV distribution system is 20KV and there are 2 new supplies (A&B) to be installed to the new building.

New building total load is around 12,900 KVA and each supply must be able to accept full load.

Upstream breakers in Main MV switchroom are 1250A and the Incoming breakers in the new building MV switchroom are 630A.

The route between switch rooms is fully ducted (with cable chambers) and the distance is around 460m.

The current cable design is for 3x1c 185 AWA cables per circuit with AWA bonded at both ends and a separate 1c x 150mm CPC.

Cables to be installed in trefoil within ducts.

Current design has the MV terminations bonded at both ends


My question is; is double ended bonding suitable for this length of run or would there be potential issues with high circulating currents?


Regards, Seansasta
Parents
  • 0
    you will get currents circulating along one armour and back down the other two, if all the armours are glanded to common chunks of metal at both ends.  Which way it flows will rotate in step with the primary phases. Make sure the votage drops and current ratings you use allow for this armour loss - the  figures in UK standards do, but I am not sure this is the case in all countries. If you only bond one outer and use insulation glands on the rest, you will introduce an earth voltage offset. It is possible to reduce this problem by swappping links between the armours around at intervals along the cables, but it is not normally worth it at 50Hz, it is easier, and at 20kV safer, to take the hit of the modest extra loss.

    previous similar discussion (for LV but the amps do not care about that....) 

    regards Mike
Reply
  • 0
    you will get currents circulating along one armour and back down the other two, if all the armours are glanded to common chunks of metal at both ends.  Which way it flows will rotate in step with the primary phases. Make sure the votage drops and current ratings you use allow for this armour loss - the  figures in UK standards do, but I am not sure this is the case in all countries. If you only bond one outer and use insulation glands on the rest, you will introduce an earth voltage offset. It is possible to reduce this problem by swappping links between the armours around at intervals along the cables, but it is not normally worth it at 50Hz, it is easier, and at 20kV safer, to take the hit of the modest extra loss.

    previous similar discussion (for LV but the amps do not care about that....) 

    regards Mike
Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines