Equipotential Bonding Explanation

[KeenPensioner]

OK......I know I'll get slaughtered for asking this question because I'm not an electrician but I'm working my way through Level 2 Diploma in Electrical Installations for no other reason than I like to learn. I've searched the web till my head hurts and watched YouTube videos by John Ward, Chris Kitchner and many others till I'm dizzy but I just don't get Equipotential Bonding. I see that the purpose involves joining together metalwork and conductive items that are or may be earthed so that it is at the same potential everywhere, normally to Gas/Water/Oil/Building. As I understand it, if an exposed conductive part becomes live through a fault and a person touches it and at the same time an extraneous conductive part that is not bonded they'll get a shock but if the extraneous conductive part is bonded they won't. Obviously I'm wrong. Can someone explain it in layman's language please. I attach an image.....surely the human will get a shock here.

 

[The Ghost]

I think you have had too much to think. You seem to have the essential understanding in your statements. What may be missing is an extraneous part can be at a different potential to the earthed metal work hence a danger of shock in the difference between the parts. However if the extraneous part is joined to existing earth you avoid that difference with the bonus of tying into the additional protection of the RCD. It is one of those things that you don't know what to ask for the right answer you are looking for. Maybe pan back from just the parts of the earthing system and look at the whole thing???

 

[kenny7askew]

if for example a fault in your metal kettle made the casing become live ie 230v and you touched this at the same time as the metal tap on your kitchen sink that is earthed it would be very dangerous,so by linking them both together via bonding makes the situation safer reducing the risk of shock

 

[Strima]

Do you understand potential difference?

 

[Des 56]

I suspect you may need to have a better picture of what happens when a potential difference exists,where it exists current can flow in a circuit
Having no potential difference,it will result in no flow

The guy is touching 2 connected parts,it should result in no flow through his body because no PD exists

 

[bigspark17]

if for example a fault in your metal kettle made the casing become live ie 230v and you touched this at the same time as the metal tap on your kitchen sink that is earthed it would be very dangerous,so by linking them both together via bonding makes the situation safer reducing the risk of shock

What?? I dont think this is a very good explanation to the op!
How is it dangersous if its bonded, your contradicting yourself..

 

[Ian1981]

Do a little research on touch voltage as well, that might help

 

[kenny7askew]

What?? I dont think this is a very good explanation to the op!
How is it dangersous if its bonded, your contradicting yourself..

i tried to explain it in simple terms for the DIY poster,i dont know how you could read what i said wrong,you do understand the difference between earth and bonding ?

 

[Wilko]

Great to hear you love to learn!
Two metal parts that you can touch at the same time may be very dangerous during a fault. The equipotential bonding makes the two locations the same potential so current won't flow. The poor chap may still get a zap, it might come via his shoes and for sure it would be much reduced.

 

[KeenPensioner]

Thank you all for your prompt responses. I understand what PD is and current flow on fault (If). I understand what Ze, Zs, PFC, ADS, OCPD, MET, etc,etc are - I even understand the adiabatic equation and have studied the maths behind touch voltage (Ut). I understand how earthing works, but I still don't get EB. kenny7askew's answer is where I'm at. If the kettle has developed a fault and the casing has become live, surely ADS kicks in? What am I missing?

 

[telectrix]

right. taking the kettle for example. the casing is earthed so is at 0 volts wrt the earth itself. the water and gas pipes are bonded, so they are also at 0V. now, if the kettle casing becomes live (230V) due to a fault the PD from that casing to earth is 230V. the water and gas pipes, because they are bonded and have minimal resistance to your kettle casing, they will also rise to 230V, or close to it. this means that if you are in contact with both kettle and bonded extraneous parts (water, gas etc.) the PD across you body will be negligible. the magic figure is 50V. ADS will kick in and then both kettle casing and pipework return to 0V. this is why we endeavour to get a high fault current to ensure that the MCB/Fuse breaks within the 0.4 secs. stipulated by BS7671.

bloody long-winded for me, that. think i need a beer.

 

[Pete E]

The casing is earthed so is at 0 volts wrt the earth itself. the water and gas pipes are bonded, so they are also at 0V. now, if the kettle casing becomes live (230V) due to a fault the PD from that casing to earth is 230V. the water and gas pipes, because they are bonded and have minimal resistance to your kettle casing, they will also rise to 230V, or close to it. this means that if you are in contact with both kettle and bonded extraneous parts (water, gas etc.) the PD across you body will be negligible. the magic figure is 50V. ADS will kick in and then both kettle casing and pipework return to 0V. this is why we endeavour to get a high fault current to ensure that the MCB/Fuse breaks within the 0.4 secs. stipulated by BS7671.

I have to say this is the bit that confuses me along with the difference between bonding and earthing...

In the kitchen example quoted we have a faulty kettle casing at 240V and lets say we have a metal sink that is also bonded to lets say a towel rail. Both of those items are supposed to be earthed as well.

Now, if you walk into the kitchen wearing your rubber clogs and touch the kettle, no problem....You are at 240V but there is no current flow as there is no path to earth. If the kettle casing happened to be already touching the sink, again no problem as it will short to earth and trip the RCB/MCB.

The problem starts if you touch the sink or some other earthed object and the faulty kettle casing, causing you to provide a path to earth via your body, current flows and you receive a shock until the RCB trips.

So as I look at it, the more metal surfaces that are bonded and earthed, the more chance there is of you touching a faulty appliance and an earth?

 

[kenny7askew]

if supposing the main earth to your house was lost because of maybe a broken earth rod or something then if the kettle became live there would be no fault path to trip the mcb,so the kettle would be live but because of bonding it would be at the same potential as your pipes ect

 

[Richard Burns]

Bonding is there to reduce the touch voltage that may be experienced during a fault, i.e. for the 0.4s before the automatic disconnection occurs. A person could die in less than 0.01s if the current is high enough.
It is also there in case of the failure of ADS to disconnect where the risk of an electric shock is much higher because there is more time to experience it and in this case bonding will keep the touch voltages below (ideally) 50V and so (potentially) safe.

 

[Pete E]

Bonding is there to reduce the touch voltage that may be experienced during a fault, i.e. for the 0.4s before the automatic disconnection occurs. A person could die in less than 0.01s if the current is high enough.
It is also there in case of the failure of ADS to disconnect where the risk of an electric shock is much higher because there is more time to experience it and in this case bonding will keep the touch voltages below (ideally) 50V and so (potentially) safe.

Thanks...

So should all bonded surfaces also be earthed?

 

[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

 

[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.

 

[ELECNEWT]

Thanks again Richard.
I had not taken account of the 500 ohm resistance connection to earth.
What is the calculation to derive proportion of fault current to 0.45 amps ?

 

[Pete E]

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

Thanks for that..I think I need to o back to TEC and refresh my Electrical Theory as I am struggling with the calculations in the second and third diagram ! lol I need to sit down with a pencil, paper and calculator and work through the maths myself....

 

[KeenPensioner]

Richard...thanks again for your patience but I don't get the maths this time. I understand parallel conductors but am I correct in this....where you draw the fault current path in the small dotted line (from just under the MET to earth) one route is 0.1 ohms for the "earth path" and the other (parallel?) path is 0.05 ohms ("bonding conductor") and 500 ohms "connection to earth. If the voltage drop is 57.5 V across this parallel circuit and you say there is a voltage drop of 0.02V over the "bonding conductor" does this not suggests a current of 0.4 amps flowing in that leg of the parallel circuit which would mean 574.6 amps flowing through the "earth path". If you can be bothered humouring me, I'd appreciate it.

 

[Richard Burns]

The calculation is not that precise, rough figures only.
The earth fault loop impedance is (0.1+0.1+0.1+0.1 Ω) = 0.4Ω, the 500Ω from any additional earth paths is ignored.
Ipf = V/R = V/Zs = 230/0.4 = 575A
If you were to calculate the parallel resistance of the extraneous part and the earth cable back to the transformer this would be ((500*0.1)/(500+0.1)=0.09998Ω which is effectively 0.1Ω to 1 dp.
Therefore in a fault 575A should flow through the circuit.
Because the extraneous Conductive part is by definition connected to earth with a resistance of less than 22,000Ω (assumed to be 500Ω here) there is a potential split of current at the MET. For a 230V supply this would then be 230/500=0.46A, because the actual voltage to earth at that point would be only 57.5V the current would actually be 0.11A but as a worst case look at 0.46A. The bonding conductor at 0.05Ω with a current of 0.46A would drop 0.02V, an insignificant amount compared to 57.5V.
The fall in current on the earth path would be 0.46A and so the earth path would take 574.54A, which is insignificantly different from 575A.

The diagram is there to show that any volt drop on the bonding conductor can be ignored for the purpose of calculating touch voltage as it would err on the side of caution. The comparative resistances are orders of magnitude different and so the worst case values are used ignoring minor variances from elsewhere. Voltages and resistances can change over time and so this is a text book calculation using specific figures.

Thanks for that..I think I need to o back to TEC and refresh my Electrical Theory as I am struggling with the calculations in the second and third diagram ! lol I need to sit down with a pencil, paper and calculator and work through the maths myself....

I have just done a quick calculation on the fly, I am not doing a full mathematical treatment so please check the maths and take account of any errors made, however the background information should allow you to understand the theory enough to make any needed corrections.

 

[ELECNEWT]

For a 230V supply this would then be 230/500=0.46A, because the actual voltage to earth at that point would be only 57.5V the current would actually be 0.11A but as a worst case look at 0.46A.

Thank you Professor Burns. I calculated 0.11A and I did not realise that you had instated 0.46A as the worst case scenario.
You must be a very patient individual to respond to such amateurs as I am.

 

[KeenPensioner]

Richard......I can't thank you enough for your comprehensive explanation. I now understand what bonding is all about and can back it up with the maths behind it. I echo ELECNEWT's sentiments above. I'm now moving on to supplementary bonding! Thanks again.

 

[johnduffell]

Maybe I'm missing something but I feel like people are mixing up ADS and EEB.
ADs is where the cases of class 1 equipment and other electric casings eg swa armour and steel conduit are all connected to the neutral at the substation (via the cpcs in the installation and on via the dnos CNE conductor, or separate CPC in tn-s or even the installation earth rod and the substation earthing.)
The purpose of ads is to remove the supply withing 0.4s or 5s of a fault, thus reducing the danger to occupants.

eeb is earthed equipotential bonding, and refers to the connection together of the main earth and any extraneous conductive parts. Those are anything likely to introduce a potential (usually thought of as earth potential, but could be other than earth in some situations with tn-c-s) from Outside the building.
So as you can see the fault inside the installation is handled by ads, and faults outside are handled by equipotential bonding.

Now I'd agree that the bonding will also reduce voltages between the water pipe and the kettle during those 0.4 seconds, but that is because the water pipe is introducing earth potential. But even so, it will be cleared within 0.4s so that is classed as sufficient for safety.

I hope that makes sense as I'm on my phone so it's basically impossible to edit. And thanks for the interesting discussion.

 

[KeenPensioner]

Richard.....I can't thank you enough for your comprehensive explanations. I "get it" after many hours (as an enthusiastic amateur) trying to figure it out. I now move on to supplementary bonding.....wish me luck. Thanks again.

 

[KeenPensioner]

Richard, thanks very much for comprehensive explanations.

 

[Richard Burns]

Thank you all for the acknowledgements, it does encourage me to respond again I am pleased that it has helped and hopefully it may help others to follow the reasoning.
I am glad that the explanation makes sense, but do always remember to check that I have it correct, I am just posting quickly on a forum and not writing a textbook.

Supplementary bonding should be straightforward now as it is just shortening the length of the cpc over which voltage is dropped so that the local area has even lower touch voltages.