Sizing of a Generators Earthing Conductor

[Knobhead]

To our OP. from you’re first post you give no figures other than “about 1000A”. That could be anything from 381KVA @220V to 19MVA @11KV, how are we to know?
As has been said there are no one size fits all solutions.

E54 has said how he would do the job which I agree with but I was bought up with earth mats which I prefer.

Wade; E54 and myself must have damn near 80 years experience between us. You ask questions on this board, have you ever known either of us ask for help? We give our time freely.
I have asked questions, for a reason, to get people to think about the theory behind accepted principles.

Give some meaningful information and we will help.

 

[old_sparky_RE]

These sites I turn up at could be a temporary set up inside another FOB or they could be put in as temporary and still be there 5years later!!
Configuration could be Master, Master, slave ...Master and slave but always prime power for the locations.

I go fora spacing of 1.8 - 2x time the depth of the rod.

400V 750kW genny at unity could pump out 1000A+ (not taking into account down rating factors such as ambient air temp, altitude and they will not be running at unity) so I'd use 4x 150mm 5core or 2x 150mm singles (MCCB wound down to compensate) 90degree Multi cable to go to the changeover switch then…… you get the idea!

So theoreticallyI have 150mm x 4 = 600mm of copper S/2 = 300mm or 150mm if I use singles.

How did you come up with a figure of 70-95mm though?

Tony –Sorry you are right I did leave a very big window open there on my earlier post. Add my experience and you will have over 100years knowledge including war, famine, flood and earthquake (I think there could be a TV show in the making here (based on the A-Team I’ll be the Murdock character) I’ll have my people call your people. OK?!).

 

[Engineer54]

Yes if you use S/2 you'll come up with some fantastic sized earthing cables...lol!! Try using the adiabatic equation and you'll get a figure somewhere around my suggested sizes. There is nothing stopping you from using a 150mm earthing conductor if you so wished, which from your figures would be the rule of thumb size to use. As i understand it, on these military out bases, they rod every building DB as well, which will further enhance your systems earthing protection.

I'd be more concerned of the overall Ra value of your generators earthing system rather than the size of conductors connecting to it. The lower the value, the larger the conductor. Don't forget, that all your gen-set's will (or dam well should do) incorporate earth fault relay's that will limit the duration of any earth fault, as well as i would hope, other cascading earth fault relays and RCD devices within the distribution system.

 

[old_sparky_RE]

Thank you E54.
Only 60Hz systems have rods at each panel (MDP, SDP & CDP) with the building and the board linked together to that local rod.
On 50Hz systems only the genny has an earth! That’s why I put as much metal into the ground as I can when my name goes against a job.
Earthfault protection kit is everywhere, even though some of these gen set were around when Moses was a lad,! But they have been tinkered with to include all the bells and whistle.

Thanks again for the direction E54…. the Abracadabra equation.