RCDs in series and garden power

[michaelsf90]

I wired a new circuit in my garden last year. One thing I wasn't sure of but figured it can't be much of an issue was having two rcds on the same circuit. So from the consumer unit I have a 2.5mm ring to a Wiska box outside. That then is a roughly 5 metre piece of 4mm swa into the shed consumer unit. That's on a 32a rcbo at the consumer unit. On the shed consumer unit is a 40a mains rcd. When testing sometimes one has tripped, sometimes both. Just wondering if anyone has ever come across it before?

Also from the shed I have a 2.5 swa to the end of the garden to feed a pond pump and filter. This is about a 25 meter cable run. When doing cable calcs will the length be from the shed consumer unit or the one in the house? The shed consumer unit is obviously getting its earth from the main earth into the house. So that's a good probably 50 metre run to the house. Another grey area I come across as well is do I need an earth rod at the shed? There are no extraneous conductive parts. No metal in the shed. No metal at the pond

 

[timhoward]

In general not a problem to have more than one RCD, not best design and can be pot luck which (or both) trip. If you can live with that, fine. There is a way to test the downstream one alone, but that is going off topic.

Also from the shed I have a 2.5 swa to the end of the garden to feed a pond pump and filter. This is about a 25 meter cable run. When doing cable calcs will the length be from the shed consumer unit or the one in the house?

Cable calcs could mean a couple of things - what did you have in mind?
Volt drop shouldn't be an issue as long as you don't have a 20 amp pump!
The total earth loop impedance can be measured at the very end of the circuit.

Thinking back nearly a year - did the DNO ever turn up to sort your earthing out, and if so are you still TNS?

My sixth sense raises a flag about whether you are already relying on the RCD for fault protection as the last 25m section adds about 0.4 ohms alone (if 3 core SWA). It all depends on the length of the house ring circuit, how far from the substation you are, the type of earthing system you have, and the MCB/RCBO rating at the shed consumer unit for the circuit that supplies the pond.

 

[michaelsf90]

I could change the rcd in the shed consumer unit to a mains switch. That way only one rcbo at the consumer unit.

The pump and filter pulls 50 watt. Its absolutely nothing. I need to do an earth loop test. I need to borrow a tester from work. I can work out r1 and r2

They did come and sort the cut out and put a new earth terminal in. It is still a tn-s supply. The cable they used was something I haven't seen before. It was a single core conductor in the middle which is the phase. Then half the armourings are neutral and half are the earth. They tested the earth and it was 0.10

So from the consumer unit to the wiska box outside I used 25 metres of 2.5 twin and earth. Probably a few metres short so call it 12 metres distance for a single 2.5. Then the 4mm swa from the Wiska box to the shed is 7 metres

From the shed consumer unit the pond supply is 25 metres of 2.5 swa off a 20A mcb. The thing I'm considering is replacing the t + e in the house to 6mm and the swa from the wiska box to shed in 6mm seeing as its off a 32A rcbo but I know it's covered now loading wise. Its more if I need an earth rod or not

 

[brianmoooore]

The 6mm T&E just to feed the 6mm SWA?
Why do you want to fit an unnecessary joint? SWA isn't hewn from solid gold; the cost over T&E is negligible in the grand scheme of things.
Run the SWA all the way, then you'll be able to dispense with the RCD at the source end, and feed from a switch fuse instead.

 

[michaelsf90]

The shed is a few metres from the utility room. So I ran twin and earth in the house only. Drilled through and put a joint box on the wall. From that joint the swa goes to the shed. I don't know how else I'd do it with it being concrete floor in the utility room as it's an extension

 

[westward10]

Your profile suggests you have the AM2 and NVQ3?

 

[brianmoooore]

I'm considering is replacing the t + e in the house to 6mm

So what does this mean, if you can't change it?

 

[timhoward]

Unless changing the shed to a 'proper' supply from the origin as Brian said I don't see an urgent reason to change anything.
It's an almost non existent load, the Zs seems to be well under 1.75 ohms for the B20 breaker even allowing for the house rfc, and it doesn't appear to be breaking any regs.
(And no, you don't need an earth rod.)

 

[pc1966]

The cable they used was something I haven't seen before. It was a single core conductor in the middle which is the phase. Then half the armourings are neutral and half are the earth.

That is 'split concentric cable' and typically used by DNOs for TN-S supplies if they provide one to replace an existing TN-S feed.

The 'concentric cable' just has the phase surrounded by the PEN (combined N & E) looking like copper armour which is what most new places on TN-C-S would be fed with.

The outer in both cases looks like armour but is sort of the reverse of SWA. In concentric cable it provides the CPC and also the safe disconnection if you stick a shovel through it where buried, but it is not intended to provide much of a physical protection as steel would, whereas the armour in SWA is predominantly for physical protection as well as the safe disconnection if ultimately penetrated, but can be used as CPC in many cases (but not all, as often its R2 is several times the R1 and not always up to PEN bonding CSA requirements).

 

[michaelsf90]

I don't know how I'd get an armoured all the way to the shed. The t and e is in a mini trunking sat just above the skirting which blends in nicely and isn't seen in majority of places behind fridge and freezer and other things. An armoured in there would be unsightly

It was good watching the dnos do their thing and the live working aspect of it. Watching them split the lead off the existing to joint onto it