Equipotential Bonding Explanation

[David M]

In basic laymans terms:-
Electrical current will flow via the least resistive path. obviously we want to avoid/minimise current flow through the human body during a fault condition when an exposed conductive part is touched.
Connecting any exposed conductive parts together (bonding) allows the current to flow through the bonding conductor rather than the person.

We test exposed conductive parts to see if there is a "potential difference" (Tel and Richard touched on this in their posts) that would be dangerous. I would advice you do some reading up on - potential difference & touch voltage at this point.

If there is a dangerous potential difference then this needs to be addressed i.e - bonding.

Hope this helps.

 

[KeenPensioner]

Thanks again.....I'm getting closer to understanding this. telectrix's explanation really helped and I can see that under fault conditions (because of bonding) both the kettle and the extraneous parts will be at the same voltage (230V). Without the bonding, only the kettle would be at 230V and there would be a PD between the two. Richard Burns also helped by explaining that ADS could take 0.4s (or fail) and a person could die in 0.01s. The last bit of the puzzle for me is that if you are in contact with both kettle and bonded extraneous part (both at 230V) will you not get a shock if you are standing on the ground? Is it because (as David M says)..."current will flow via the least resistive path"

 

[Ian1981]

Also if your protective measure is ADS then protective bonding is crucial if applicable to the installation ie they are extraneous conductive parts accessible to touch within the property/installation.

 

[telectrix]

just done an inspection at a house. no bonding visible to gas or water at the entry points, but a wander lead gives each point of entry 0.20 ohms to MET. the gas is bonded to the cold water via the boiler.I,m considering a C3. any thoughts?

 

[Devonchris]

The last bit of the puzzle for me is that if you are in contact with both kettle and bonded extraneous part (both at 230V) will you not get a shock if you are standing on the ground?

Bonding and eathing in this context is referring to situations inside a building. A floor is not normally in contact with the earth. Although in some buildings it can be which is why any earth grid or screen is set in the floor, or a wet bathroom floor can be in contact with a radiator pipe, which is why special locations apply. However services or steelwork can be set in the ground giving an earth path which is why they are bonded.

In out door situations, swimming pools, hot tubs, caravans and elv points etc. where you could be in contact with a conductive part in a fault situation and with the earth, particularly if the ground is wet, then there would be a potential difference. Which is why an earth rod is required which is, effectively, bonding the local earth.

 

[Ian1981]

just done an inspection at a house. no bonding visible to gas or water at the entry points, but a wander lead gives each point of entry 0.20 ohms to MET. the gas is bonded to the cold water via the boiler.I,m considering a any thoughts?

I would C3 it if the connection isn’t visible for inspection and testing yes, having confirmed the said pipework is connected to the MET through testing it and that I was sure I wasn’t picking up parallel paths anywhere like through the cpc connected to the boiler.
I personally don’t code it’s location being further than its point of entry maybe just a comment on the report.

 

[westward10]

Never assume something is there and likely to remain there, this I discovered today. Circuit over 50 yrs old, radial to a single socket, conduit the cpc. Carry out efli test which is okay but we here cracking and then see some sparking at high level. Ladder out and find the conduit is unfixed laying across a metal waste pipe. Lift conduit up and wedge in a piece of wood, Zs now 127 ohm. Conclusion, waste pipe earthing the circuit. Unless you are 100% certain, never assume and Code as necessary, in my case it was good fortune it was noticed otherwise none the wiser would we have been.

 

[Midwest]

just done an inspection at a house. no bonding visible to gas or water at the entry points, but a wander lead gives each point of entry 0.20 ohms to MET. the gas is bonded to the cold water via the boiler.I,m considering a C3. any thoughts?

Would that be with the installation isolated, main earth disconnected and then the points checked?

 

[Richard Burns]

Thanks again.....I'm getting closer to understanding this. telectrix's explanation really helped and I can see that under fault conditions (because of bonding) both the kettle and the extraneous parts will be at the same voltage (230V). Without the bonding, only the kettle would be at 230V and there would be a PD between the two. Richard Burns also helped by explaining that ADS could take 0.4s (or fail) and a person could die in 0.01s. The last bit of the puzzle for me is that if you are in contact with both kettle and bonded extraneous part (both at 230V) will you not get a shock if you are standing on the ground? Is it because (as David M says)..."current will flow via the least resistive path"

I drew a diagram for another purpose some time ago to try to provide some information about how differing potentials can arise in different situations in a property and how a house is considered to be isolated from earth. I attach this here as a possible useful reference.

 

[telectrix]

Would that be with the installation isolated, main earth disconnected and then the points checked?

yes.

 

[Wilko]

I would C3 it if the connection isn’t visible ... and I was sure I wasn’t picking up parallel paths anywhere like through the cpc connected to the boiler.

If there was no 10mm g/y leaving the MET then the low number must be due to boiler CPC only? If so I'm C2 and a beer please.

 

[Midwest]

yes.

I know its stating the obvious, but I previously didn't realise to do that. So just for clarity.

Was the incoming gas/water service plastic

 

[ELECNEWT]

Would any of the mathematically inclined members be willing to show some calculations for the following?

1. The touch voltage when the main bonding conductor to the water pipe in Keenpensioner’s diagram is the prescribed value of 0.05 ohms.
2. The touch voltage when the main bonding conductor to the water pipe is an unacceptable value of 5 ohms.

 

[Richard Burns]

Would any of the mathematically inclined members be willing to show some calculations for the following?

1. The touch voltage when the main bonding conductor to the water pipe in Keenpensioner’s diagram is the prescribed value of 0.05 ohms.
2. The touch voltage when the main bonding conductor to the water pipe is an unacceptable value of 5 ohms.

The actual resistance of the bonding conductor is immaterial as it will be carrying minimal current during a fault and so the voltage drop over the bonding conductor will be effectively zero.
The 0.05Ω is just to check that the connections are sound and the conductor continuous.
The voltage drop over the cpc of the faulty circuit, whilst it is carrying fault current, back to the point where the bonding (of any type) joins it is the determining factor for touch voltage.

 

[ELECNEWT]

The actual resistance of the bonding conductor is immaterial as it will be carrying minimal current during a fault and so the voltage drop over the bonding conductor will be effectively zero.
The 0.05Ω is just to check that the connections are sound and the conductor continuous.
The voltage drop over the cpc of the faulty circuit, whilst it is carrying fault current, back to the point where the bonding (of any type) joins it is the determining factor for touch voltage.

Thank you Richard