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?