Equipotential Bonding Explanation

[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

 

[KeenPensioner]

Thank you all again - maybe I'm not asking the right question so I've attempted to draw what I mean, bearing in mind all the advice that has been given regarding equipotential bonding, ADS activation times etc. The image tries to show a fault in a kettle where the casing becomes live and develops a 230V potential as does the tap due to the bonding (am I correct in that?). The fundamental question for me is if stick man touches both kettle and tap at the same time as the fault develops and before the MCB/RCD trips, will he get a shock?

 

[Devonchris]

No he won't get a shock. Yes the tap will become live.

The effect of bonding is too make all the exposed metal work one big conductor, with little resistance, flowing to earth, the path of least resistance. So any two parts of it will be at the same potential.

Its the same as putting your test leads on two seperate points of the same live conductor. The meter will not register a voltage as it won't register a difference.

 

[sinewove]

Am I missing something,or was the requirement to bond sinks and steel tables dropped in an earlier edition for safety reasons in cases of contact with a live appliance and the sink?

 

[Pete E]

No he won't get a shock. Yes the tap will become live.

In that diagram, wouldn't the voltage from the kettle case effectively fall to zero if it has a direct low resistance path to earth?

 

[Richard Burns]

Just to confuse further I have done another diagram to show touch voltages with and without bonding.
In the diagram with the example values the touch voltage would be 57.5V and for the resistance of a human body at 1000Ω the current through the body would be 5mA, only just perceptible, therefore someone would not feel a significant shock, perhaps tingling.

 

[static zap]

Main Earth Conductor ... now MET - Main earth terminal !

 

[ELECNEWT]

Just to confuse further I have done another diagram to show touch voltages with and without bonding.
In the diagram with the example values the touch voltage would be 57.5V and for the resistance of a human body at 1000Ω the current through the body would be 5mA, only just perceptible, therefore someone would not feel a significant shock, perhaps tingling.
View attachment 40726

Richard, what a superb and comprehensive illustration!!

If you can spare the time, could you calculate the current distribution for your third diagram to demonstrate why the resistance of the main bonding conductor, whether it is 0.05 ohms or 5 ohms, has no effect on the touch voltage?

 

[Midwest]

Richard, what a superb and comprehensive illustration!!

Agree, just need somebody to explain it to me now

 

[KeenPensioner]

Just to confuse further I have done another diagram to show touch voltages with and without bonding.
In the diagram with the example values the touch voltage would be 57.5V and for the resistance of a human body at 1000Ω the current through the body would be 5mA, only just perceptible, therefore someone would not feel a significant shock, perhaps tingling.
View attachment 40726

Richard, thanks very much, I understand the maths behind this now. I've spent ages on this and eventually the penny has dropped thanks to all the responses here but in particular your excellent diagram. One point (a typo or me?)...for a resistance of 1000 ohms and a touch voltage of 57.5 volts is the current not 57.5 mA?

 

[Richard Burns]

Quick sketch (which i never worked out before!) to show that there is minimal volt drop over the bonding conductor because the current through the bonding conductor is practically zero compared to the fault current on the cpc.
The extraneous part is not really involved in the current path at all.
(Better check the maths though!)

Richard, thanks very much, I understand the maths behind this now. I've spent ages on this and eventually the penny has dropped thanks to all the responses here but in particular your excellent diagram. One point (a typo or me?)...for a resistance of 1000 ohms and a touch voltage of 57.5 volts is the current not 57.5 mA?

"Do not do calculations late at night without thinking", it would be 57.5mA so an "ouch that hurt" shock and muscle contraction after 0.4s. Below 50V touch voltage the shock should be perceptible without adverse effects for the duration of the fault (0.4s)
However these are just random resistance numbers used for convenience, real life may be very different.