Why are main bonding conductors so big?

[apprentice87]

Hi Sellers,

"Next door may have a lower impedance on their neutral but i highly doubt the impedance would be less by the time it goes through my bonding and water pipe through their bonding and down their neutral? Therefore it would go down my neutral"

Hmmm, not so sure; You are of course right up to a point, but houses just a few doors away from you might all be bonded to the same metallic services as your house, but fed electricity from a different feeder cable [with different impedance to yours] but all connected to the same supply transformer, and therefore, there is no knowing what happens as the load on the network changes, but be assured, problems with overloaded neutrals caused by different supplies bonded together, are a well known problem for the DNO's hence the TOTAL ban on a steel building having more than one TNCS supply.

john...

 

[SLIPSHOD & SLAPDASH]

Are my kids safe in the bath then?.......I've got metal taps.And what about their electric toothbrush??

Young mother electrocuted by live tap as she ran a bath in her family's new home | Mail Online

 

[ADS]

Young mother electrocuted by live tap as she ran a bath in her family's new home | Mail Online

Quite - but with our regs we tend to try to deal with 'single fault' conditions..............there's not much you can do when the heater is faulty, has a damaged flex, the wrong size fuse, no RCD protection, no supplementary bonding and the installation itself has no earthing system............bit of a different scenario, do you not think???

This lot are talking about currents coming from houses down the street and zapping you - hardly what happened to the woman in the story.

In fact, I'm still waiting for news examples (links) of people being electrocuted because of the dreaded 'lost' neutral on a TN-C-S......just a couple will do.

 

[Doc Hudson]

In fact, I'm still waiting for news examples (links) of people being electrocuted because of the dreaded 'lost' neutral on a TN-C-S......just a couple will do.

Why? There is far more to electrical faults than just people being electrocuted. Lost neutral can and does cause excessive currents down other paths which can damage appliances, circuit cables, protective conductors etc if they are not of a suitable CSA. A lot of electric shocks do not get reported or written in newspapers, we had a member the other day saying he had received a shock whilst at work, but I doubt there would be any newspaper or H&S links anyone to post for you and I doubt every TNCS fault and or its resulting symptoms gets reported. It maybe more helpful to post your opinions on the topic as to "Why are main bonding conductors so big?" as your contributions so far have had little constructive input. unless you are seriously implying that you have your electric toothbrush bonded?

D'you think??

Are my kids safe in the bath then?.......I've got metal taps.And what about their electric toothbrush??

Ahh, that's what I love about this forum - when things get explained to us properly..........so, it's 10mm bonding so that it can carry the current from the rest of the street when the supply neutral breaks

.I wish they'd just told us that during training.....it's so simple when you think about it.

As I said, the mind boggles!

The mind boggles

 

[ADS]

Why? There is far more to electrical faults than just people being electrocuted. Lost neutral can and does cause excessive currents down other paths which can damage appliances, circuit cables, protective conductors etc if they are not of a suitable CSA. A lot of electric shocks do not get reported or written in newspapers, we had a member the other day saying he had received a shock whilst at work, but I doubt there would be any newspaper or H&S links anyone to post for you and I doubt every TNCS fault and or its resulting symptoms gets reported. It maybe more helpful to post your opinions on the topic as to "Why are main bonding conductors so big?" as your contributions so far have had little constructive input. unless you are seriously implying that you have your electric toothbrush bonded?

Oh, hello again, Doc, let me explain myself.

The tone of this thread was set in the first post - inaccurate and misleading - the thread then went off on a completely different tangent.

In truth, from some of the answers, I thought people were answering the wrong thread.

You're right, though, my posts were irrelevant drivel that did not come close to answering the ops question -

but certainly in keeping with the rest of the thread.

As you seem to like the quote button so much, perhaps you could quote us ONE post from this thread that actually answers the question in the title...............you'll actually be lucky to find a post that's relevant to the question!

 

[noonaldinho84]

I always thought that is why Bonding conductors are so big ? ?:|

 

[rich.250]

Fault current what flows in the bonding conductors in a fault condition.

Just think of the PeFC you measure then you will realise why....

ohms law shows higher resistance means less current flows, this is why TT can usually have smaller conductors.

Another reason for this is to keep below the required 0.05 ohms. We have to achieve from the conductor. I.e if it is over a 27m run in 10mm it will need to be up'd to comply.

 

[ADS]

Bonding conductors are not sized to carry fault current.

They are used to equalize the potential difference between exposed/extraneous conductive parts.

The reason we try to achieve 0.05 ohms is because this is considered 'negligible resistance' - in other words, it doesn't matter......it might as well be 0 ohms.

What the negligible resistance ensures is that a voltage appearing on one piece of exposed metalwork will be 'mirrored' on any metalwork bonded to it............so no potential difference.

That is the whole purpose of 'bonding' - nothing to do with carrying fault currents or clearing faults................although, by the nature of how they are connected to extraneous metalwork, there will be some current flowing down them, but that is not their purpose.

The reason 'bonding conductors' on TT installations can be smaller in size than on TN systems is because you are allowed to calculate the size of the earthing conductor using the adiabatic equation.

As there is a lower PEFC going to flow in a TT system, then you can have a smaller 'earthing conductor'.

As the 'protective bonding' conductors are then based on the size of the 'earthing conductor' (TT), they too, will be a smaller size.

Where PME conditions apply, the 'bonding conductors' are selected in relation to the supply neutral..............this being you protective earth and neutral (PEN), conductor.

So, as I said, this talk of fault currents from down the street, lost neutrals and 'bonding conductors' carrying thousands of amps of fault current is all drivel......it's nothing to do with it.

 

[steptoe]

Bonding conductors are not sized to carry fault current.They are used to equalize the potential difference between exposed/extraneous conductive parts.

The reason we try to achieve 0.05 ohms is because this is considered 'negligible resistance' - in other words, it doesn't matter......it might as well be 0 ohms.

What the negligible resistance ensures is that a voltage appearing on one piece of exposed metalwork will be 'mirrored' on any metalwork bonded to it............so no potential difference.

That is the whole purpose of 'bonding' - nothing to do with carrying fault currents or clearing faults................although, by the nature of how they are connected to extraneous metalwork, there will be some current flowing down them, but that is not their purpose.

The reason 'bonding conductors' on TT installations can be smaller in size than on TN systems is because you are allowed to calculate the size of the earthing conductor using the adiabatic equation.

As there is a lower PEFC going to flow in a TT system, then you can have a smaller 'earthing conductor'.

As the 'protective bonding' conductors are then based on the size of the 'earthing conductor' (TT), they too, will be a smaller size.

Where PME conditions apply, the 'bonding conductors' are selected in relation to the supply neutral..............this being you protective earth and neutral (PEN), conductor.

So, as I said, this talk of fault currents from down the street, lost neutrals and 'bonding conductors' carrying thousands of amps of fault current is all drivel......it's nothing to do with it.

total toss, you just prove yet again you do not understand the fundamentals of earthing,

if 'all' they were there to do was equalise the potential under 'normal' conditions then 1mm would be sufficient,

how large is the potential going to be once next doors fault appears on your metalwork?

this is why they are sized so large, to clear this extreme potential difference quick enough and safely.

 

[rich.250]

Bonding will still take fault current in a fault!!

And yes negligible impedance is 0.05.

 

[ADS]

total toss, you just prove yet again you do not understand the fundamentals of earthing,

And 'total toss' back at you - considering we're discussing 'BONDING'

Please try to remember that 'Earthing' and 'Bonding' are two seperate entities...............you can actually have one without the other.

For example:

Run a sub-main to a second CU in a seperate room with NO extraneous conductive parts - complete with CPC for earthing, but no bonding required.

Bonding metalwork in an earth free location - lots of bonding conductors, but no earth required.

They are seperate entities with there own specific uses.

---------- Post Auto-Merged at 21:39 ---------- Previous post was made at 21:26 ----------

Bonding will still take fault current in a fault!! And yes negligible impedance is 0.05.

OK, my house installation earthing system is TN-C-S and the bonding up to the latest spec.

Bearing in mind that the RCD will trip if there is a fault on the consumer side, we'll have to use the old 'lost neutral'.

Let's assume a load at the time of 30 A.

We lose the neutral and my metalwork PD rises to 230v to earth - under normal conditions, everything would just stop working - but we have our 'earthed', (via the gas pipe), bonding.

Lets be really generous and say that my incomming gas pipe has an Ra of 20 ohms............how much 'current' is going to flow down there due to losing my supply neutral?

Lets take the loss of neutral to the end of the street - and assume that each of 10 houses was using 30 A each......300 A load and no neutral.

If I was the only house with a gas bond at 20 ohms, how much current is going to flow down my 10mm bonding conductor now?

 

[steptoe]

And 'total toss' back at you - considering we're discussing 'BONDING'Please try to remember that 'Earthing' and 'Bonding' are two seperate entities...............you can actually have one without the other.

For example:

Run a sub-main to a second CU in a seperate room with NO extraneous conductive parts - complete with CPC for earthing, but no bonding required.

Bonding metalwork in an earth free location - lots of bonding conductors, but no earth required.

They are seperate entities with there own specific uses.

---------- Post Auto-Merged at 21:39 ---------- Previous post was made at 21:26 ----------

OK, my house is TN-C-S (PME) and the bonding up to the latest spec.

Bearing in mind that the RCD will trip if there is a fault on the consumer side, we'll have to use the old 'lost neutral'.

Let's assume a load at the time of 30 A.

We lose the neutral and my metalwork PD rises to 230v to earth - under normal conditions, everything would just stop working - but we have our 'earthed', (via the gas pipe), bonding.

Lets be really generous and say that my incomming gas pipe has an Ra of 20 ohms............how much 'current' is going to flow down there due to losing my supply neutral?

Lets take the loss of neutral to the end of the street - and assume that each of 10 houses was using 30 A each......300 A load and no neutral.

If I was the only house with a gas bond at 20 ohms, how much current is going to flow down my 10mm bonding conductor now?

wrong,

you have stated you have PME, so the previous points about TNCS are not valid arguments for your post,

this is where folks get mixed up, TNCS is NOT always PME.

and you have upgraded your house to the 17th edition with an RCD too, [nice ]

 

[ADS]

wrong,you have stated you have PME, so the previous points about TNCS are not valid arguments for your post,

this is where folks get mixed up, TNCS is NOT always PME.

Yes..........well mine is, so rap up.......I'm talking to rich.250 - you're not turning this thread into a TN-C-S/PME discussion again.......I'm sure everyone knows where you stand.

 

[steptoe]

ADS,

you really need to understand that TNCS and PME are not the same thing, and stop trying to convince other people they are.

you also need to try and learn the fundamental basics of why earthing systems in this country are the way they are.

 

[ADS]

......and you have upgraded your house to the 17th edition with an RCD too, [nice ]

If you're taking the 'P', you need to look up the definition of RCD.

In case you have a problem doing that......RCD is a generic term covering both RCCBs and RCBOs

 

[rich.250]

Fair point, I may have wrote before reading the whole post. So I do apologise.

Although fault current will flow in the bonding conductor in fault conditions, all be it the amount, which will vary on many circumstances.

The primary selection will be to do with impedance as I said.

I'll leave you lot to it

 

[ADS]

ADS,you really need to understand that TNCS and PME are not the same thing, and stop trying to convince other people they are.

you also need to try and learn the fundamental basics of why earthing systems in this country are the way they are.

OK, Steptoe, just for you I've altered my original post.......now can we move on??

 

[green-hornet]

The mind boggles

 

[Dan Martin]

The bonded earth conductor has to be of a sufficient csa to carry the fault current long enough to trip the main C/O fuse of a fault in the event of the shared N/E not present. Too small a conductor will not provide a sufficent fault current to trip the overcurrent protection.

BTW PME and TNC-S are the same. There are different methods of bonding the neutral & earth is all.

A TN-C service differs where the neutral and earth stay combined

 

[Andy™]

BTW PME and TNC-S are the same.

no they are not

PME is always TNCS, but TNCS is not always PME