The wisdom of equipotential bonding

[alexcarnes]

I'm probably displaying my ignorance here, but fortunately I'm not one of those people who can't stand to be laughed at!

I've been thinking a bit about one of those things I've tended not to dwell on. The technical reason for equipotential bonding is obvious - if you create equipotential zones, such that everything rises to the same potential in the event of a fault, then we reduce the risk of shock from touching simultaneously accessible conductive parts.

Very well. But doesn't it run the risk of simply making lots of conductive parts live to Earth instead of just a few? So instead of a bit of cable tray over there being live, they all are. So unless you're wearing thick rubber shoes...!

Doesn't it risk solving one problem and creating another?

 

[Rockingit]

Otherwise known as “Electricity was perfectly safe until we started earthing everything!”

It’s a bit of an all or nothing situation - you either build a system where safety is afforded by guaranteeing there’s no possible way to make a circuit under a fault condition or you build one that’s as efficient as possible in making that fault as big as it can be to make manufactured protection work. As there’s no money to be made in the first option we’re stuck with the second.

 

[alexcarnes]

Otherwise known as “Electricity was perfectly safe until we started earthing everything!”

It’s a bit of an all or nothing situation - you either build a system where safety is afforded by guaranteeing there’s no possible way to make a circuit under a fault condition or you build one that’s as efficient as possible in making that fault as big as it can be to make manufactured protection work. As there’s no money to be made in the first option we’re stuck with the second.

I hadn't thought about the financial implications!

Thanks for that. So I'm not completely wrong then.

Edit: although isn't the idea that supplementary bonding reduces the shock risk in the event that manufactured ADS fails or isn't feasible for some reason? It seems to me that it risks causing everything to have a potential to Earth instead of just something.

 

[westward10]

Are you referring to earthing or bonding?

 

[alexcarnes]

Are you referring to earthing or bonding?

Equipotential bonding, where we cause everything to rise to the same potential.

 

[westward10]

So we have three extraneous parts in close proximity two of which are bonded together and to earth. The third is not and becomes live, what happens should you touch the live one and one of the others?
Connecting all parts together essentially makes them a whole single part assuming the resistance of those bonds are of a sufficient low resistance.

 

[Dustydazzler]

Americans link their Ground to Neutral at the Panel

 

[alexcarnes]

So we have three extraneous parts in close proximity two of which are bonded together and to earth. The third is not and becomes live, what happens should you touch the live one and one of the others?
Connecting all parts together essentially makes them a whole single part assuming the resistance of those bonds are of a sufficient low resistance.

Right, but if the fault current is insufficient to actuate your ADS, then you now have three live parts instead of two.

It's this business of them all rising to the same potential - potential to Earth. If your ADS does its stuff then you don't need any equipotential bonding do you. The idea certainly of supplementary equipotential bonding is to try to make it so that the fault doesn't create a circuit with you in it. But isn't there just an increased risk of you touching something live and making a circuit with the conductive mass of the Earth back to the transformer?

 

[alexcarnes]

Americans link their Ground to Neutral at the Panel

I suppose it's much the same as TN-C-S electrically - send the fault current back to the transformer via the neutral.

 

[westward10]

If you have three parts bonded together you essentially make it one part.
If you omitted bonding then my scenario in #6 becomes extremely hazardous.
Your last sentence I assume you mean touching a live part to an earthed part and of course this is a hazard which is why we can put in place additional RCD protection as an alternative.

 

[Mark Wright]

I've wondered this. If there is a fault and it doesn't trip for whatever reason. Everything Earthed and Bonded is linked at the MET potentially meaning every exposed and extraneous (spelling) is live.

Now we are limiting our changes because probably the most common conductor? That has a good link to earth the water pipe is bonded and so at equal voltage. But little Timmy touching the the metal socket front during the fault in his bare feet?

 

[westward10]

All things being equal if your earthing and bonding is sufficient then along with circuit protection ADS is satisfied and little Tommy should be perfectly safe.

 

[alexcarnes]

I've wondered this. If there is a fault and it doesn't trip for whatever reason. Everything Earthed and Bonded is linked at the MET potentially meaning every exposed and extraneous (spelling) is live.

Now we are limiting our changes because probably the main conductor? That has a good link to earth the water pipe is bonded and so at equal voltage. But little Timmy touching the the metal socket front during the fault in his bare feet?

Yes, you can see what I'm saying can't you. If your ADS does its stuff then happy day, but if it doesn't then absolutely everything rises to the supply voltage relative to Earth.

I do understand the principle of equipotential bonding, and these days of course, more often than not, ADS will save the day. However...

 

[Mark Wright]

Well yeah before RCDs the MCB has to be at x3 - X5 and doesn't offer protection to life so yeah I've wondered this.

I'm guessing the main potential danger is touching the pipe at the same time as the fault is considered much more dangerous than other faults because of its good link to earth.

 

[davesparks]

Equipotential bonding, where we cause everything to rise to the same potential.

We only bond items which introduce an earth potential into the building.

We don't bond isolated bits of metal.