UK Bonding of extraneous conductive frame

[Tequila]

Yes that's fine.

Remember to ensure that the earth/CPC and armour of the cable is correctly earthed at the source end, and is NOT connected to the rod, or anything that can be touched at the TT end.

Thanks Julie, one other scenario I'd like to run past you regarding similar situation with an aluminium greenhouse with an outdoor socket in it, and a plastic florescent strip light.
I'm thinking same solution. Rod at greenhouse to turn socket and light only into TT system. Note the now 3 installed rods at MET,pond and greenhouse are 10+ Meters from each other.

 

[Julie.]

Thanks Julie, one other scenario I'd like to run past you regarding similar situation with an aluminium greenhouse with an outdoor socket in it, and a plastic florescent strip light.
I'm thinking same solution. Rod at greenhouse to turn socket and light only into TT system. Note the now 3 installed rods at MET,pond and greenhouse are 10+ Meters from each other.

Keep everything in the greenhouse class 2 and you don't need to bother.

The light sounds class 2 anyway - it's just a matter of making sure you don't have anything class 1 used.

 

[mainline]

Ok think I'm getting there. If I placed an earth rod at the pond and made a TT with low reading <200ohms and bonded metal frame to this with 4mm earth cable..

Has your metal frame a lower resistance to earth than your earth rod has ?

 

[Tequila]

Has your metal frame a lower resistance to earth than your earth rod has ?

I've not tested the frame, but unlikely as not as deep...although there is 3 connections to earth with the frame..

 

[mainline]

I've not tested the frame, but unlikely as not as deep...although there is 3 connections to earth with the frame..

It could be that you didn't need the earth rod.
What's the Ra ?

 

[cliffed]

Any pictures of this metal framework

 

[cliffed]

There are actually a whole host of considerations behind the need to/to not bond , so it pays to understand these in order to understand why things are done the way they are.

So, let's imagine your house has its supply from a substation 100m away.

Ignore the line and neutral, when you consider the earth cable, that actually is at the potential of the earth at the substation - not necessarily the same as the ground potential at the location of the house.

Now imagine that your water pipes are metallic throughout, including the underground supply, which connects to your neighbour. Unfortunately your neighbour has a fault, a live conductor has contacted the water pipework.

If you were now to grab your kettle (at zero voltage due to the earth/CPC connection to the substation) and turn the tap (now at circ 230V due to the fault in your neighbour's house) you would receive this 230V!

What's worse is that this has nothing to do with your own supply, in this case, your kettle is off, so disconnecting the line or/and neutral will do nothing - the danger is coming in to your property via the pipes (an ECP)

This is the principal reason why we bond the water/gas etc at the point of entry - in this scenario, the bonding at your (and everyone else's) property safely sinks the voltage away, likely tripping the supply in your neighbour's property.

Of course there are many reasons why the pipework or local ground in general raises in voltage due to faults in supply cables maybe at high voltage etc etc this bonding keeps you safe by bringing all the potential sources of an induced voltage together at the same voltage.

However now the pipework is interconnected between properties then in the case of a fault between the substation and a group of properties (PEN fault) there could be substantial current flowing in the bonding conductor - hence the seemingly large size.

In your case, of a remote ECP the situation is kind of reversed, if one was stood on the ground local to the pool, and a fault elsewhere was to raise the potential of the ground to say 200V if you now touched the metalwork which is connected to zero volts -again you would receive 200V - and it's nothing to do with your actual 230V supply.

So bonding metallic stuff out in the wild when it doesn't have a good connection to the local ground would be a bad idea

(As an aside, this is why class 2 equipment is preferred, and why most power tools are now made class 2, when used outside if there was an induced voltage the class 1 CPC connection would present the same danger as described above)

If the metalwork does have a good connection to the local ground then the local ground will be brought to zero volts , but a substantial current could flow.

With TT you would be at that 200V when stood at the pool, but so would any local metalwork, in this case you need to ensure that contact with the earth connection from the substation is prevented.


The key issue about the whole bonding thing, is most of the time the danger isn't from your own supply, so disconnecting that won't solve anything.

Of course one still needs proper protection for your own supply in the case of a fault actually to do with your own supply.

Sorry bit of a long and boring post!

Julie that’s a great explanation & we are talking of an open PEN … if this is the case then all metalwork on that system in a fault condition would be @ 230v including the Earth connections, during that fault condition.
If that is the case then there would no potential difference & assume no shock situation.
In normal fault conditions the OCPD would operate

 

[Julie.]

Julie that’s a great explanation & we are talking of an open PEN … if this is the case then all metalwork on that system in a fault condition would be @ 230v including the Earth connections, during that fault condition.
If that is the case then there would no potential difference & assume no shock situation.
In normal fault conditions the OCPD would operate

That is one possible scenario, however it's not a likely one, it is fairly possible in remote areas, it can only occur if either the bonding isn't in place or all services are plastic, and even then only if the broken PEN is between the property and the last earh rod (as PME)

In general, the likelihood is that the PEN is upstream of a rod (PME) giving a limited current back via the earth - in this case the property would experience a reduced voltage L - N/E - it's why there is a suggestion that rods are fitted at the property supplementary to the PME (as Ireland).

However the most likely scenario is a Broken PEN, the bonding links the neutral in the PEN-less properties to the water/gas service to another property where the PEN is sound. In this case, imagine if two properties, both on the same phase /PEN happen to use their 10kW showers - the one sound PEN property would see the return (normally neutral) current in their bonding. (~90A)

Two showers is unlikely, however two EVCP is fairly likely and although only 64A it could be for many hours.

This as you point out , if it occurs at a remote type property could result in the earth/neutral raising to 230V - that would be the car bodywork! (Also any outdoor metalwork which is earthed via the CPC or supplementary bonding applied inappropriately.

Hence why PEN faults are a big concern with EVCPs

 

[Tequila]

It could be that you didn't need the earth rod.
What's the Ra ?

Finally had time to do a 2LH test earth rod reading 36ohms and frame is 65.3ohms so yes looks like I couldve saved my hammering arm and just used the frame.. but as the rod is a lower reading I'll just use that.

 

[Tequila]

Yes that's fine.

Remember to ensure that the earth/CPC and armour of the cable is correctly earthed at the source end, and is NOT connected to the rod, or anything that can be touched at the TT end.

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?

 

[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