Earth rods to reduce Ze

[davesparks]

Surely you wouldn't need a 200A sw fuse upstream of the two 100A ones, it would just need to be a switch wouldn't it?

 

[Kamikaze]

Hypothetical here.... not telling you how to do it.

You have a 140KVA supply
Zs can only be achieved on your side if you fusing to BS88(2) is 125amp or less
You can run 2 x 100amp supplies but issue is they need a common isolation upstream
Fitting a 200amp SW/F upstream would not comply because of Zs failure as 0.15 cannot be acheived.

What will happen in an earth fault then if your Zs is 0.22 instead of 0.15... your cabling could be subject to thermal damage due to the extended duration of the fault, so what are your solutions here?

You can either reduce Zs/Ze which is probably not possible or maybe increase the CSA of the cable proportionally to allow for the extended tripping time... now Ive never needed to do this myself and its hypothetical so would like others input here but if you can get around the issue of the thermal damage to a correctly sized cable then you have taken measures to counteract the poor zs and as its probably for the sake of a few meters of tails that can be housed in steel trunking for added protection then Id be happy with the set-up.

Even I might wear my tin hat for this one lol

Why not use an upfront 200A 60947-2 MCCB instead of an isolator? Of the top of my head I think the maximum earth loop for one of those is 0.22-0.25 ohms-ish.

 

[Darkwood]

Ill retract some of post 40 .. the insulation would be damaged too and increases cable size would not restrict this effect.

@Kamakaze... not got the table to hand but may be an option.
@Davesparks... also maybe an option even Id have to check that as im used to having Busbar set ups so Ive always had a main sw/f set-up regardless, industrial set-ups tend to increase loads with years of additions and id rather have consumer fusing going than service fuses not really an issue domestic hence common to rely on service fuses to protect tails.

 

[davesparks]

Ok, I'm pretty sure the book of minimum standards only requires it to be a main linked switch. If it is immediately followed by 2x 100A switchfuse then it'll be ok (until they add another submain from the origin)

 

[Engineer54]

The DNO's fusing Zs requirements TBH are their issue if they comply or not but from experience the DNO work to different regs and often in confliction with our own, as is a small supply in the scope of things then full discrimination cannot be achieved and little benefit will be had by derating to 160A customer side.

If the customer has a 200amp (approx' 140KVA) allowance then he should be able to use it all, an O/L will correctly take out the customer fusing first if 200amp BS88's (2) against 200amp BS88 (3) leaving only S/C faults an issue and as I mentioned you can't achieve full discrimination anyway on these small supplies so pointless derating customer upstream fusing.

.

They would still need to ensure that Zs is of a sufficient value to take out their service fuses, it'll be, or could be a very expensive repair exercise on their part if they don't!!

Been trying to get data from fuse manufacturer's web sites, typically they give just about all the information you're NOT looking for!!! lol!! Having to go through third party sites to circumvent the restrictive bamboo internet wall, it just takes too long, so i've given up!! lol!!