Bonding of Extraneous pergola frame

[rattlehead85]

Need a bit of advice to clear up some uncertainty. We have a TN-S earthing system supply at the house. Main gas and water are bonded using 10mm cable. Supply running to shed at bottom of garden is 3 core 6mm using house earthing system protected at the house by 32a mcb with RCD metal board in shed. We have a metal frame pergola bolted to the ground which has an ip65 socket and light on it. The frame tests as extraneous back to M.E.T in house. Can this be bonded using 6mm back to the board in the shed and use the existing earthing conductor contained within the swa feeding the shed to get the bond back to the m.e.t of the installation or will i need to run a dedicated 10mm directly from the m.e.t to the pergola frame meaning i need to trench dig back to the house. Advice appreciated.

 

[Dave OCD]

As your supply cable is 6mm and the fault current would be limited by the 32a OCPD [worst case scenario] I'd be happy with a 6mm bond personally but others may disagree.

 

[Leesparkykent]

Need a bit of advice to clear up some uncertainty. We have a TN-S earthing system supply at the house. Main gas and water are bonded using 10mm cable. Supply running to shed at bottom of garden is 3 core 6mm using house earthing system protected at the house by 32a mcb with RCD metal board in shed. We have a metal frame pergola bolted to the ground which has an ip65 socket and light on it. The frame tests as extraneous back to M.E.T in house. Can this be bonded using 6mm back to the board in the shed and use the existing earthing conductor contained within the swa feeding the shed to get the bond back to the m.e.t of the installation or will i need to run a dedicated 10mm directly from the m.e.t to the pergola frame meaning i need to trench dig back to the house. Advice appreciated.

The bonding conductor must have a cross-sectional area of not less than half the CSA of that required of the earthing conductor of the installation, with a minimum size of 6mm2. The CSA need not exceed 25mm2 for copper or the equivalent for other metals.

Edit: forgot to add any CPC complying with reg 544.1.1 can be used as a bonding conductor.

 

[Deleted member 9648]

It is pointless to bond a metal frame in an external location. The whole point of bonding is to create a zone of equal potential, in an external location the frame is surrounded by the general mass of earth,it is not possible to create a zone of equal potential therefore bonding will be ineffective. In the event of a fault causing a rise in potential of bonding and conductive parts the risk of a shock to a person in contact with the frame would be increased,not reduced.

 

[rattlehead85]

It is pointless to bond a metal frame in an external location. The whole point of bonding is to create a zone of equal potential, in an external location the frame is surrounded by the general mass of earth,it is not possible to create a zone of equal potential therefore bonding will be ineffective. In the event of a fault causing a rise in potential of bonding and conductive parts the risk of a shock to a person in contact with the frame would be increased,not reduced.

Would it not be a case then that seeing as the pergola itself has items of electrical equipment fixed to it the frame becomes an extraneous conductive part which could introduce a potential under fault conditions.??

 

[davesparks]

As your supply cable is 6mm and the fault current would be limited by the 32a OCPD [worst case scenario] I'd be happy with a 6mm bond personally but others may disagree.

Main bonding should be sized according to the requirements of the incoming service, not reduced at any point in the installation

 

[rattlehead85]

Main bonding should be sized according to the requirements of the incoming service, not reduced at any point in the installation

And as far as actually bonding the frame as it tests as extraneous yes or no?

 

[DNO]

You wont need to get your fork out for digging, this is a simple SNE/TNS earth return system, 6mmsq is adequate bonding.

 

[rattlehead85]

You wont need to get your fork out for digging, this is a simple SNE/TNS earth return system, 6mmsq is adequate bonding.

Would you bond it or not. Getting mixed opinions on it.

 

[malcolmsanford]

I'm with WP on this and would not bond it. Would you bond an external say cast iron lamp post.

Bonding within a shed is understandable if your extending the equipotential zone from the house to that shed, and the contained shed is either metal framed, or has metalic pipe for services, but it would be impossible to extend the zone to a piece of metal in the middle of earth.

This is the virtually the same scenario concering metal fire escapes that have EM lights on them, or cables clipped to them. I have seen loads of these Bonded/Earthed. Should they be?

 

[Deleted member 9648]

Would you bond it or not. Getting mixed opinions on it.

No.
If you connect this metal frame to the earthing/bonding arrangements of the entire installation a fault anywhere on the installation may introduce a dangerous potential onto all metal work connected to the system,including the pergola. Within a building this is not an issue because a person is effectively insulated from the general mass of earth by the normally electrically insulating floor.The purpose of bonding is to prevent a PD between conductive and extraneous conductive parts within the building.
Externally a PD on a metal frame would be very dangerous to a person in contact with the frame and the ground,hence IMO bonding the frame increases the shock risk considerably. Neither is it considered a conductive part as it is not part of the electrical installation so earthing it should not be required. It is up to the installer to make sure cables and accessories are installed in a manner which prevents any possibility of a fault to the frame.

 

[SS-2015]

I had a similar thing when having to instal some 110v sockets on some steel work benches. (Bolted to floor)
What decided it for me was the fact that when I tested between known earth and benches I got a reading of 0.9M ohm, if they had been totally insulated (i.e meter maxed out) then I wouldn't have bothered but in this instance I decided that if a fault did occur I would want the fuses to trip asap so decided to bond.

 

[Dave OCD]

Main bonding should be sized according to the requirements of the incoming service, not reduced at any point in the installation

Yes I realise that but took the view that as electrical equipment was being attached to a metal structure then the structure should be earthed, 6mm would be more than adequate in such circumstances as the maximum earth fault current would be limited and of very short duration as the aforementioned equipment was RCD protected.

 

[rattlehead85]

It's clearly evident from the responses to this thread that there is a clear difference of opinion wether the structure should be bonded or not.

 

[Richard Burns]

I had a similar thing when having to instal some 110v sockets on some steel work benches. (Bolted to floor)
What decided it for me was the fact that when I tested between known earth and benches I got a reading of 0.9M ohm, if they had been totally insulated (i.e meter maxed out) then I wouldn't have bothered but in this instance I decided that if a fault did occur I would want the fuses to trip asap so decided to bond.

This is an interesting response.
When you measured the resistance to earth you got a value that would limit any current attempting to flow through the bench to well below the perceptible limit of 5mA, so this would indicate that the bench was not an extraneous part even by the most rigorous standard.
You also state that you would want the fuses to blow as fast as possible and this can only be achieved by reducing the resistance of the earth fault loop path so as to maximise the fault current.
However you decided to bond the bench because your measured resistance was too low and to cause the fuses to blow even though bonding would have no significant effect on the fault current.
Please help me to clarify the reasons for your decisions.

 

[SS-2015]

This is an interesting response.
When you measured the resistance to earth you got a value that would limit any current attempting to flow through the bench to well below the perceptible limit of 5mA, so this would indicate that the bench was not an extraneous part even by the most rigorous standard.
You also state that you would want the fuses to blow as fast as possible and this can only be achieved by reducing the resistance of the earth fault loop path so as to maximise the fault current.
However you decided to bond the bench because your measured resistance was too low and to cause the fuses to blow even though bonding would have no significant effect on the fault current.
Please help me to clarify the reasons for your decisions.

Firstly I'll apologise - should have said 0.09M.

But just interested in you comment that bonding doesn't have any significant effects on fault current?

 

[Richard Burns]

I was just starting to draw a diagram to explain when I found this one I prepared earlier and modified slightly to fit your situation.
Hopefully this demonstrates the limited current that flows though bonding conductors in the case of a fault, and the minimal change bonding would make to the EFLP.

 

[SS-2015]

I get this and would almost certainly apply in the OP's situation but in my situation, this was a workshop with no Gas and water present, therefor there would be no parallel paths, it would be relying purely on main earth and subsequent CPC's and due to benches being bolted to floor and not 100% insulated from ground I felt it necessary to bond.

 

[rattlehead85]

I get this and would almost certainly apply in the OP's situation but in my situation, this was a workshop with no Gas and water present, therefor there would be no parallel paths, it would be relying purely on main earth and subsequent CPC's and due to benches being bolted to floor and not 100% insulated from ground I felt it necessary to bond.

The difference between my situation and this one mainly being that my Pergola is outside bolted to the ground whilst yours is internal where the floor is most probably insulated from the general mass of earth.

 

[SS-2015]

The difference between my situation and this one mainly being that my Pergola is outside bolted to the ground whilst yours is internal where the floor is most probably insulated from the general mass of earth.

Normally you'd be right, but as this was reinforced concrete industrial floor, I'm guessing that the bolts holding bench are touching rebar somewhere....

anyway wasn't my intention to have your thread hijacked.