You also have a good earth at the substation I hope to operate that 10A setting  promptly - do we consider a 9 amp fault to be safe forever at that level  that's the after diversity demand of a small semidetached house, as earth leakage ?

An interesting challenge.

On the odd occasion I have had to set up installations with nested earth fault protection, and I will say never a fuel station (!),  I have used a rule of 2 or 3 - so 30mA instant is backed by 100mA 0.1 seconds, in turn backed by 300mA and delay of 0.2 to 0.3 seconds so far this has always worked without a hitch, but I am not sure how much one can squeeze the settings up a bit before things start simultaneous tripping.

In your shoes I'd be asking what the various levels of disconnection do, and how serious the collateral damage of the next layer up tripping as well would be - if I knew the designers well I may ask why they think they  need so many levels of 'defcon' .

All but a lowest and fastest levels of  trip should indicate an attack on the infrastructure, damage to buried submains and so on, rather than a faulty component, and it may be that in such a special  case a wider shut down is OK.

It may be worth a call to the maker of the earth fault relays  about the discrimination.

Let us know how you get on.

Mike

I've probably not pictured the setup entirely correctly, but it sounds suspiciously like there's an RCD on both ends of the same cable in some instances (200A ones at least) - arguably the 2nd of these isn't strictly necessary and could either be omitted or at least need not discriminate reliably with its upstream sibling. So maybe you only have three or four tiers to achieve discrimination between rather than five - which might be a little easier.

   - Andy.

Hi Lyledunn,

Given I have specialised in filling station electrical installations for the last 40 years and sit on the Tech Committee that produces the tech standard ( Blue Book) for that unique industry, then please send a pm if you have specific questions. On a typical UK filling station normally three levels of time/earth leakage current cascading, after all it will be a TT earthing system, so depending on the technical spec of the client? usually the principal protection device is set at between 1A and 3A earth leakage (Not usually any higher than that) and a 0.5S time delay or thereabouts. The submain protective devices usually set at 300 - 500mA earth leakage with a 150 - 300mS time delay and then your final circuits 30 - 300mA with instantanious delay.

Yes, cost is expensive................

Regards GTB