Sub-main set up

[Davisss]

Submain set up

• Davisss
• 17 Feb 2014
• 1

Hi there, I have just been asked to quote for a job and am need of a little input from you guys if possible. As the picture shows it is a series of sub-mains and i'm wondering what the best set-up could be with out involving RCBOs. It is a TT system and would require earth electrodes in all locations due to the distance between the locations.
What are your thoughts to the set-up as shown in the diagram??

Your input is much appreciated.
Dave

 

[ricicle]

A 100mA TD RCD won't necessarily discriminate with a 30mA RCD. You may have to go to 300mA TD.

Also a 30mA TD RCD will serve no useful purpose either for your park home sub-main.

 

[AndyPRK]

An oven element went at the weekend and both a 500ma and 30ma rcd went!

 

[ricicle]

An oven element went at the weekend and both a 500ma and 30ma rcd went!

Was the 500mA time delayed ?

 

[AndyPRK]

No.

 

[SimonH2]

Why not split off the garage from the house ? As it is, a fault in the garage or park home would take out the house as well - which I'd imagine would be "very annoying". So split off the garage supply on a separate TD RCD - or could the submain be run without upstream RCD if the cable type is chosen properly and the earth rod is low enough impedance ?

Series connected RCDs will not discriminate unless they have substantially different tripping times - ie the upstream one is time delayed. Potentially a slowly developing fault might trip the more sensitive one without the other, but in general a fault will create a leakage current that is substantially larger than the tripping current of the RCD - the RCD does NOT limit the magnitude of this current. Eg, you may well get several hundred mA - which will trip both the 30mA and the 100mA. If the 100mA upstream is time delayed, then the 30mA one should have tripped (cutting off the fault) before the 100mA one times out. But if the time delay isn't enough, then allowing for the contacts on the faster one actually opening, it may have reached the "point of no return" before the fault is removed.

 

[flyingsparks]

A 100mA TD RCD won't necessarily discriminate with a 30mA RCD. You may have to go to 300mA TD.

Really?? I was always under the impression that a 10mA TD RCD should discriminate against a 30mA?

 

[JohnW2]

A 100mA TD RCD won't necessarily discriminate with a 30mA RCD. You may have to go to 300mA TD.

I find that hard to believe. A non-TD RCD of any IΔt ought to operate before a TD one of any rating, shouldn't it?

I agree that a 100mA non-TD one can't be guaranteed to discriminate against a 30 mA non-TD one (or a 100mA TD one against a 30mA TD one), but that's not what you said.

Kind Regards, John

 

[Davisss]

Thanks tor the input guys. So, simonh2, are you saying from the meter take the tails into henley blocks, then a set going into a time delayed 100ma rcd unit for the submain to the workshop. The workshop fuseboard could then be a dual 30ma rcd board. Then from the henley blocks I could take another set directly into the main house fuseboard. This could just be a dual 30ma rcd board. What would be the best option then for the submain going to the parkhome?

 

[EFLImpudence]

Are you forgetting short circuit protection for the sub-mains?

 

[Davisss]

Of course not. 63amp mcb for the submain.

 

[JohnW2]

An oven element went at the weekend and both a 500ma and 30ma rcd went!

Was the 500mA time delayed ?

No.

Is not one of the points/problems probably that if the L-N imbalance is extremely large, many many times greater than the IΔt of the higher rated RCD (as with a low impedance L-E fault) (both being non-TD), that higher rated one will operate so rapidly that even one with a very low IΔt 'may not have time' to operate beforehand and hence provide discrimination?

Kind Regards, John

 

[EFLImpudence]

Of course not. 63amp mcb for the submain.

Is (or was prior to Henlys) that in the house CU protected by one of the RCDs?

 

[Davisss]

At the moment the supply comes into the house, service fuse, meter then into an old voltage operated earth trip. This will be replaced by a couple of time delayed rcds. One for the house board and one potentially for the submain to the workshop which will also be suitably fused down. I hope this is a bit clearer. I'm still scratching my head for the best solution for the supply to the park home from the workshop. TT systems do make it a bit more difficult to set up!! FYI the Ze at intake is 3.81 Ohms.

 

[SimonH2]

Thanks tor the input guys. So, simonh2, are you saying from the meter take the tails into henley blocks, then a set going into a time delayed 100ma rcd unit for the submain to the workshop.

Yup - split them off (with, as pointed out) the correct overcurrent protection etc. That way, when you trip the RCD from the garage, you won't plunge the house into darkness as well. It's bad enough tripping the garage out and leaving yourself stumbling in the dark - but if you have to go back to the house in the dark then that's even worse.

Consider the third "site" as well. Ideally arrange that so it doesn't go through the garage RCD. Either split the incoming submain, or take it off an non-RCD way in the garage CU (and provide local RCD protection at it's CU). I'm assuming the layout doesn't lend itself to running this as another submain from the house.