Cascaded time delayed RCDs for discrimination on Farm system

I'm planning to rework the distribution system on the farm and have a supply in a meter house near the road, some distance from the nearest farm buildings. Basically I wish to provide a 300mA time delayed RCD at the meter house with an extended time delay so that I can pick off sub-systems along the route of the 25mm SWA main (backbone) feed cables which runs down the site and have tap-off points for each building, employing "type S" 100mA time delayed RCDs at each building distribution board incomer, followed by 30mA general purpose (fast) RCD protection for socket outlets etc. on the appropriate final circuits. Normal type S RCDs disconnect in typically less than 0.2 seconds I understand. General purpose disconnect in 0.04S at 150mA leakage current. The general purpose RCDs are freely available, the normal time delayed type S 100mA units also easy to source. Where I am finding a problem is sourcing a 300mA RCD with a time delay greater than the 100mA units have, so discrimination can be assured as I don't want to take out the whole system if an earth fault occurs on one of the distribution system cables protected by the 100mA type S units. Without going into fantastically priced speciality units does anyone know of reasonably priced 300mA RCD with a delay disconnection time between 0.2 and 1 second as allowed by 411.3.2.4 and suitable for a

100A TPN service. The service is TN-C-S so I need to convert to TT, not extend the suppliers earth into the farm systems.
Reply to
cynic
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If you had a 300mA type S at the supply head end, that that will ensure adequate indirect contact protection, equipment, and fire protection for all of the sub systems. You only need the worry about direct contact protection in the individual building consumer units.

Perhaps you should look at a slightly different architecture. Rather than having a three level cascade, reduce it to two. Have the 300mA type S on the main head end, then then independent non delayed 100mA and 30mA RCDs on either separate CUs or in a CU with two discrete incomers - one RCD handling each side - but not cascaded.

Failing that, go for the 17th edition super delux solution and use all RCBO protect in each building with an ordinary main switch incomer. (that still leaves the flexibility to drop in an MCB or cartridge fuse holder to run a submain should you need one)

The MK 6400S and family claim max trip time of 450ms, and less than

300ms on 5xIn. Sadly, exact time delay details are a bit scarce:

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The service is TN-C-S so I need to convert to TT, not extend the

Are there not requirements where you need 10mA trip protection in farm environments as well?

Reply to
John Rumm

The French use much higher trip current RCDs for distribution than over here. Trying to discriminate just by using a large difference in current ratings isnt ideal, but it can frequently work, if not all the time.

These 2 are just for interests sake really... In principle its also quite possible to make your own high trip current RCD using relays, but I dont think that would be usful for you. The other possible is to forego RCD protection on some of the distribution by using armoured cable and a good enough earth, like a Ufer earth. Not exactly trivial though!

NT

Reply to
NT

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I was hoping to keep the existing Hager distribution boards and replace the incoming isolators with 100mA type S RCDs and RCBOs where sub 20A socket outlets are supplied. Using a longer delay head end RCD would have been the simplest option I hoped. Thanks for the information about the MK units. I will get in touch with their tech department on Monday. Since asking around I have also been advised that RS market a unit which might be suitable but at a higher cost of course. Thanks for the help.

Reply to
cynic

Given the BS7671 requirement for a 300mA head end RCD on farms it is surprising that a more versatile time delayed version is not listed as readily available, especially one with sufficient time delay to discriminate with a normal type S or with an adjustment available. It seems uneccessarily complex and expensive going for a distribution network scale leakage detection system. I could I suppose use the suppliers earth to protect the backbone armoured cable which is for the most part underground but there is always the risk of future workers not under my control who might take the TN-C-S armour sheath earth into the vulnerable areas. John Rumm has made a useful suggestion which I will follow up.

Reply to
cynic

ISTM that with the RCD at the head end, and the Hager boards left with an incomer switch + RCBOs, then you have the protection you need anyway without an additional RCD in the incomer section.

Well it would be from RS ;-)

Reply to
John Rumm

That would be a viable option, but you would need to ensure the cable armour is isolated from all the distribution boards. I warning notices on the cable and dis boards might help re-enforce the point.

Personally I would go with no 100mA RCDs at all, and 30mA RCBOs on the circuits where you need them. That way you will discriminate with most faults being cleared by the RCBO since you are unlikely to have much in effect directly protected by the head end 300mA RCD (probably the odd sub main etc).

Reply to
John Rumm

If I went down that route I would terminate the armoured cables with the TN-C-S earth connections into plastic boxes and link the i/c and o/ g backbone cable sheath earths then come out of the plastic box with meter tails as offtakes to the local distribution boards. I could then install sub-meters to monitor the individual building usage. This would be a useful addition if I rent out one or more of the sheds at a future date. For the cost saving by not using a 300mA type S at the head end I could purchase a few certified kWh meters. There will also be at least two 32A sockets for welders so 100mA non delayed RCDs needed there. Ho hum time for further thoughts.

Reply to
cynic

Yup, using a plastic box also avoids the issue of having a PME earth touchable outside of its equipotential zone.

Sounds like it would be a worthwhile enhancement. You could always go through a master switch after the submain tap off, and then leave the meter out for the moment. Should you decide you need one, its then a simple job to isolate the one building to fit the meter.

Is their leakage to great for a 30mA RCBO? (not looked but you might be able to find 100mA RCBOs). Failing that a split load CU with a 100mA RCD "side" for such circuits.

Indeedy!

Reply to
John Rumm

If you want to isolate the PME earth via a "plastic" SWA gland then search for the CCG glands. Nickel-plated brass base & cone nut, with a tough plastic shroud that screws up tight to the enclosure and hand locking seal nut on the rear. Nothing metal can be touched.

You can argue it was tightened by hand so can be undone without the use of a tool, but pretty immaterial. If you find assembly is difficult (the plastic shroud will not screw up against the washer on the enclosure) then simply back off the cone-nut & re-tighten. UK SWA is a little different to other countries and thus some of ours can be at the edge of specification (same with any gland).

Cheap and off the shelf compared to the "true" all plastic SWA glands which are impossible to find and probably some min-qty.

Reply to
js.b1

Handy to know, ta.

Reply to
John Rumm

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