UK Bonding of extraneous conductive frame

[Tequila]

TNC-S main supply with 16mm swa supplying garage consumer unit from main consumer unit in house, then 4mm swa supplying pond equipment through pond fuse box. (Pond cable run approx 40m)
Pond has a metal unistrutt frame and cover over pond equipment, this frame has its support legs concreted into the ground making then extraneous.
How is this best to be bonded? PME supply so has to have min 10mm bonding conductor, Can't use pond supply cable armour as not equivalent to 10mm, Can't use cpc of cable as only 4mm. Are these the correct assumptions?
Do I only have 2 options?
1) 10mm protected earth cable connected to extraneous pond frame all the way back to garage earth terminal giving me 10mm bonding conductor back to supply. OR
2) earth rod at pond, and turn pond supply into a TT system. So 4mm swa earth conductor disconnected and insulated, along with the armour at pond. And 10mm earth cable from new earth rod at pond to cpc connection of pond fuse box and pond frame?

 

[Julie.]

Julie, hope you had a good Christmas.. ive been having more thoughts on my TT earthing arrangement at my pond. The pond supply is protected by a single 30ma RCD back at the solar/battery backed up supply DB in the garage.. which trips at 66.2 ms when rcd test carried out at pond fuse box. This rcd is the only shock protection. Should the rcd fail to trip with fault introduced I'll have circa 7 amps of earth fault current flowing through my earth rod at 36ohms which will not trip the pond supply breaker which is 16amps at the pond end 20amp at the garage supply end. I guess I do have the individual fuses on the pond equipment themselves for example pond uv light which on a 2amp fuse so this would blow within 400ms? How can I make this more robust if needed? Another rcd at the pond end?

I kind of share the same concerns, when an RCD is present as additional protection then I am quite comfortable, but like in this case - the rcd is fault protection (411.4.204/411.5.2 sort of thing) just having this one device just doesn't feel enough.

By the regulations of course it is sufficient, the imbalance would be detected by the RCD, so providing it works all is good.

However, in general for ADS you have the MCB/fuse on the circuit itself, and if that doesn't operate, then the upstream main fuse will eventually operate, this along with the inherent reliably of fuses (and MCBs to an extent), plus the RCD for additional protection gives a really comfortable feeling that a fault will be cleared by at least one of the three things there.

I just don't get that when you have just the one device, especially as RCDs just haven't the same inherent reliably as other devices.

The standard solution is as used for TT installations, you have a higher current 300ms RCD upstream, and a 30mA instantaneous RCD on the circuit itself. In your case as they would be both on the same circuit, they could both be standard 30mA inst RCDs , either, or both could operate and the use of two would just provide redundancy in the event one failed to operate correctly.

But as indicated above it is actually compliant with the one RCD for fault protection

 

[Tequila]

I kind of share the same concerns, when an RCD is present as additional protection then I am quite comfortable, but like in this case - the rcd is fault protection (411.4.204/411.5.2 sort of thing) just having this one device just doesn't feel enough.

By the regulations of course it is sufficient, the imbalance would be detected by the RCD, so providing it works all is good.

However, in general for ADS you have the MCB/fuse on the circuit itself, and if that doesn't operate, then the upstream main fuse will eventually operate, this along with the inherent reliably of fuses (and MCBs to an extent), plus the RCD for additional protection gives a really comfortable feeling that a fault will be cleared by at least one of the three things there.

I just don't get that when you have just the one device, especially as RCDs just haven't the same inherent reliably as other devices.

The standard solution is as used for TT installations, you have a higher current 300ms RCD upstream, and a 30mA instantaneous RCD on the circuit itself. In your case as they would be both on the same circuit, they could both be standard 30mA inst RCDs , either, or both could operate and the use of two would just provide redundancy in the event one failed to operate correctly.

But as indicated above it is actually compliant with the one RCD for fault protection

My fuses at the pond are glass quick blow fuses so guess they offer better disconnection times but think I will definitely consider a second 30ma rcd at the pond for redundancy. But for additional protection I think if I do any kind of maintenance in and around the pond I will isolate the whole lot from the garage.
Just so I'm understanding correctly with my TT system at the pond and rcd failure and local mcb and fuses not operating im right in saying the fault current will just continue to flow through the mass of earth to transformer and be unnoticed until shock recieved at the pond equipment?

 

[mainline]

My fuses at the pond are glass quick blow fuses so guess they offer better disconnection times but think I will definitely consider a second 30ma rcd at the pond for redundancy. But for additional protection I think if I do any kind of maintenance in and around the pond I will isolate the whole lot from the garage.
Just so I'm understanding correctly with my TT system at the pond and rcd failure and local mcb and fuses not operating im right in saying the fault current will just continue to flow through the mass of earth to transformer and be unnoticed until shock recieved at the pond equipment?

Yes, if the fault is L to E.
L to N would cause the Ocpd to operate.

You would have a current of around 9 amps with rod and parallel paths connected

 

[Tequila]

Yes, if the fault is L to E.
L to N would cause the Ocpd to operate.

You would have a current of around 9 amps with rod and parallel paths connected

Ok so assuming the body resistance of 1000ohms and resistance in parallel with earth rod a dangerous current would flow through the body if class 2 pond equipment it touched during fault!'

 

[mainline]

Ok so assuming the body resistance of 1000ohms and resistance in parallel with earth rod a dangerous current would flow through the body if class 2 pond equipment it touched during fault!'

No, not with class 2 appliances, as they are double insulated.

 

[Tequila]

No, not with class 2 appliances, as they are double insulated.

Apologies I meant class 1 metallic casing

 

[cliffed]

My fuses at the pond are glass quick blow fuses so guess they offer better disconnection times but think I will definitely consider a second 30ma rcd at the pond for redundancy. But for additional protection I think if I do any kind of maintenance in and around the pond I will isolate the whole lot from the garage.
Just so I'm understanding correctly with my TT system at the pond and rcd failure and local mcb and fuses not operating im right in saying the fault current will just continue to flow through the mass of earth to transformer and be unnoticed until shock recieved at the pond equipment?

Why would the RCD fail … yes it can happen but then it may not.
Without it you have no protection on the TT System