Sizing of a Generators Earthing Conductor

[old_sparky_RE]

I’m goingto be installing some quite large Generators (1000 Amps plus on each phase) in the next few days will I need to size the EC to Table 54.7 or 54.8?
I’m leaningto 54.7 as I think that would be safer, but am I correct or just wasting copper?

 

[Knobhead]

What does it say in the design specification?

if you dont have a specification then you should employ an engineer that knows what he's doing. Asking a bunch of randoms on an internet site clearly shows you're out of you're deph.

 

[Engineer54]

Those tables in BS7671 are basically for compliance to installations much smaller than the system your talking about here. As Tony stated, you won't be installing these 1 MVA (or thereabouts)generators without drawings and specifications relating to the installation requirements, so don't second guess.

 

[Deleted member 26818]

I'd probably use the adiabatic equation.

 

[Engineer54]

I'd probably use the adiabatic equation.

Not if there is a specification you wouldn't!! lol!!

 

[Deleted member 26818]

Not if there is a specification you wouldn't!! lol!!

You're quite right, I wouldn't, unless I had reason the believe the specs were wrong.
If I did suspect something though, I wouldn't use a one size fits all table.

 

[old_sparky_RE]

Saying I wouldn’t use that really doesn’t help !!!!
Supply will usually be KVA 500 or 750 kw Gen sets slaved together.
Set up isto a 4pole change over switch, Main Distribution Panel, Sub Distribution Panelthen into Consumer Distribution Panels. The feed cable is usually between120mm-300mm ran in parallel if need be. There are no drawing as such ... thisis what’s going on here.... power it up.
The time I've been out here I don't think I've seen anything bigger that 90mm for a EC.
All I'm asking for is a pointer gents .... go and look in BS xxxx or go towww............
All that will happen is a 10 foot rod will get put in and if you’re lucky a bit of 25mm earth tape will be put between it and the genny. There will be no resistivity calculations of the soil etc a 10 foot rod will be it. The genny put on a plastic sheet a 4inch wooden bund put round it and the genny mech starts it up.Job done walk away. I’m trying to make it better.
TONY –Saying I’m out of my depth is insulting. Not knowing an answer is not out of depth.To me it seems you want others to think you know the answer but it’s a black art and cannot be shared, well fine things will carry on, nothing will improve or be made safer as “I’m out of my depth”.
This site is locked out for security in an hour so I won’t be able to reply again till tomorrow morning.
Thank youfor reading.

 

[ayjay]

bury the whole thing in a hole

 

[Knobhead]

OK, I take it you’re another one in the military.

Have fun.

 

[wade88]

Think he makes a reasonable point. Saying, "go ask an engineer" is all well and good, especially when a lot of people on this forum profess to in fact BE engineers, then just sneer at posts and refuse to provide any help, instead mock the question and resort to the old faith response, "clearly you dont know what your doing".

How incredibly resourceful.

If this isnt a forum to seek advice anymore, and instead just a entity to polish each other egos, then people need to get out doors a little more.

 

[Deleted member 26818]

I take it you're REME, and there was me informing someone else that we had no REME as members.
Are you stating that the 10 foot earth rod is going to be the main and sole method of earthing the installation the genny is supplying i.e. the installation will be TT?
Or is it the case that a TN-S earthing conductor will be used for the installation, but the genny is just being earthed to the mass of the earth?

Hold up Wade, the OP has not really explained themselves.
There was no mention in the OP that there would be multiple Gensets connected together, and there's been no mention as to what earthing system is intended to be used for installation?

As such the OP is basically saying, I'm not telling you what the earthing system is or is going to be,
but is asking: should I use the table for TN-S/TT or the the one for TN-C-S (PME)?

 

[old_sparky_RE]

How dare you Sir, I shall send my man round to beat youSir!!

REME, REME. No Sir; A Gentleman of The Corps of RoyalEngineers (retired).

There could be multiple gen sets at a location, I neverusually know until I get there.
Some if not most Gen sets are to a single Ground rod (1gen set - 1 rod 10foot long). Multiple gen sets bonded together but still 1 rod to each set. This seems to be the standard to any size genny 20kw to 750, I’ve never investigated on larger sets as it’s not my job too, but I will now!

If I can get in first I dig in a ring, Furse weld the joints, attach 4 rods to the ring (welded) and then attach the gennys.Earthing is to the general mass of the planet.

Ra is always below 20 ohms except on 60Hz where they dotheir own thing.

Distribution from the Genny will be TN-S (5wire) throughout the system.
Hope this clarifies things a bit more Spin?

TONY- No Ex-Military 22years. Why do You have a problemwith people who have served their country, seems like you may do? Did you tryto get in and get turned away, is that why?
Do you have a solution, a point of reference or any help for me or others that may come along later? Or are you just banging away like an empty drum!
I have fun every day! Do you?

 

[Engineer54]

Well sir, your correct in what you would like to do for the earthing of these larger gen-sets. ie, ring with at least 2 diagonal or 4, one on each corner. But remember, they need to be a distance away from the other rods (at least 1 1/2 X the rod length between each rod -zone of influence) On a 750 KW Genny i'd go for a 70 or even 95mm earth conductor ring, connecting these rods together, with a cad link off the ring to the N-E point and frame and another to the output termination/connection box... Are these Genny's stand-by, or prime power set's?? Temporary, semi temporary/semi permanent, or Permanent??

 

[Clive P]

Hi,

Old sparky must be right, as I'm also ex RE; and we're always right.

Hope you all take this in the spirit it is meant.

Regards.

 

[Deleted member 26818]

I'm saying nothing, and not even thinking of a word that rhymes with bongo.

 

[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.