Will bonding cause an MCB to operate?

[Ceraunic]

If bonding is there to limit the touch voltage that would suggest that
the actual bonding connections will not cause the MCB (or RCD) to trip
The CPC is there to operate the MCB during an earth fault, and that should operate in a fast time
and during that time the bonding lifts everything to mains voltage


If you had an immersion heater, and the pipe work
was connected to the MET via bonding connections, but the CPC for the circuit
was not connected to the MET, would the MCB or the RCD operate, if the live touched
the copper pipe work.
I suspect the RCD might? but not sure about the MCB

All cpcs and bonding are connected together at the MET
so the fault will flow through all these connections
would that not take the place of the missing CPC for the immersion?

 

[JohnW2]

If bonding is there to limit the touch voltage that would suggest that the actual bonding connections will not cause the MCB (or RCD) to trip

I presume that you are talking about 'main bonding'. The (only) purpose of that is to prevent there ever being a significant potential difference between the 'earth' potential of the installation (MET & CPCs, hence exposed-conductiveparts of equipment) and any 'extraneous-conductive parts' (e.g. water/gas supply pipes) which enter the premises from outside. Provided the installation has an earth, the presence or absence of main bonding has no consequence in terms of whether or not MCBs and RCDs will trip.

The CPC is there to operate the MCB during an earth fault, and that should operate in a fast time ....

Indeed.

.... and during that time the bonding lifts everything to mains voltage

during that time the voltages of the installation's 'earth' (MET/CPCs) (including anything bonded) will become very close to one another, at some voltage (relative to true earth) between zero and full mains voltage.

If you had an immersion heater, and the pipe work was connected to the MET via bonding connections, but the CPC for the circuit
was not connected to the MET, would the MCB or the RCD operate, if the live touched the copper pipe work. I suspect the RCD might? but not sure about the MCB

If there were main bonding connecting the pipework to the MET, then that would be effectively the same as having a CPC connecting the immersion to the MET - so both RCDs and MCBs would operate as one would normally expect.

All cpcs and bonding are connected together at the MET .... so the fault will flow through all these connections ... would that not take the place of the missing CPC for the immersion?

Quite so - that's what I just wrote above. In the situation you postulate, the connection from MET to immersion would be via the pipework and boding conductor, rather than at least some it being via a CPC, but the connection would still be there - so, as I wrote, everything would work 'as expected'

 

[ericmark]

Bonding is so multi faults will not cause two exposed metallic objects from have a voltage between them. If the supply on feeds one item for example a shaver then it would be safer not to have bonding. But most transformers supply many things, so we use bonding to cause auto disconnection in the case of a fault.

So a MCB is a duel device, both thermal and magnetic, the latter is much faster, but is set 5 x (B), 10 x (C) or 20 x (D) to the thermal device, so an immersion heater (3 kW) is likely supplied with a 16B MCB which will need 16 x 5 = 90 amps to trip, so 230/90 = 2.56 Ω with 5% for safety that's 2.44 Ω if using earth rods that would be hard to achieve to earth, so only option is a RCD. But still working with 2.44 Ω live to live or line to neutral.

In the days of fuses, if we got is slightly out, it was not so bad, it took a few mS longer to disconnect, but with a MCB the difference between magnetic disconnect time and thermal disconnect time is huge, so we need to use a loop impedance tester to ensure with a short circuit it will activate the magnetic part. But that's live to live, line to earth we can use the RCD so much less it required to trip it.

 

[JohnW2]

Bonding is so multi faults will not cause two exposed metallic objects from have a voltage between them. If the supply on feeds one item for example a shaver then it would be safer not to have bonding.

That statement might be confusing to those who are not 'in the know', since I presume you are talking specifically about an 'isolating transformer', the output of which is 'floating' (neither side connected to earth) - which, in UK installations, is usually only seen in 'shaver sockets'. Hence ....

But most transformers supply many things, so we use bonding to cause auto disconnection in the case of a fault.

Yes, but the DNO transformers which supply installations, hence supply 'many things', DO have one side connected to earth, so it makes no difference whether they are supplying one or many things.

So a MCB is a duel device, both thermal and magnetic, the latter is much faster, but is set 5 x (B), 10 x (C) or 20 x (D) to the thermal device, so an immersion heater (3 kW) is likely supplied with a 16B MCB which will need 16 x 5 = 90 amps to trip, so 230/90 = 2.56 Ω with 5% for safety that's 2.44 Ω if using earth rods that would be hard to achieve to earth, so only option is a RCD. But still working with 2.44 Ω live to live or line to neutral.

Again, I think this might all confuse people. For a start, your arithmetic, and basis of your calculations, could do with some improvement. Firstly, 16 x 5 is 80, not 90. Secondly, in calculating the maximum Zs which would be guaranteed to result in magnetic tripping (in the face of a zero impedance L-CPC fault) you should take Cmin (0.95, even though it should really be 0.94 ) into account and calculate that figure using a voltage of 218.5 V (230 x 0.95), not 230 V. The correct calculations would therefore have led to a minimum current for guaranteed magnetic tripping (In x 5) of 80 A, hence a maximum Zs of 2.73 Ω (218.5 / 80).

Thirdly, and perhaps most importantly, although what you say about 'using earth rods' is correct, it would probably be desrable to point out that this is true of any circuit in a TT installation - so, adequate fault protection of every/any cuircuit in a TT installation would have to be provided by an RCD.

....But still working with 2.44 Ω live to live or line to neutral.

Even I am confused by that one, so good luck to those who know less than I do What do you mean by "live to live" - do you mean "phase to phase" in a 3-phase installation, "Line/Phase to Neutral" or what? .. and what does the sentence as a whole mean?

In the days of fuses, if we got is slightly out, it was not so bad, it took a few mS longer to disconnect, but with a MCB the difference between magnetic disconnect time and thermal disconnect time is huge ....

I don't think that even "a few" (or even "a good few"!) seconds would make much difference (it's incredibly improbable that someone would be 'touching the wrong things' at the very moment an L-E fault arose), and currents approaching the magnetic trip threshold will thermally trip an MCB in well under 15 seconds (these curves being for B-curve MCBs) ...

 

[ebee]

It is often said that "Bonding and Earthing are two different things"

Well YES but then NO.
(sounds like Little Britain?).

Earthing (Circuit Protective Conductors a.k.a C P C) is in place to connect metal bodies of electrical appliances etc to "Earth" (our installation MET = Main Earthing Terminal) and we assume (incorrectly) that it would be a dead short in the case of an earth fault and therefore blow a fuse or trip a breaker or trip an RCD of some type - it might trip more than one of them at the same time or it might only trip one but either way it is job done so all well ).

Bonding (Main Bonding) is in place to connect pretty much everything in touching distance with a low impedance (resistance) because it is intended to be an "Equalising Conductor" therefore everything you might touch is at all times pretty much the same potential and will pretty much keep it all to the same voltage until the Circuit Protective Device has tripped , and at all times.
We try to keep "True Earth" to that same voltage too . Zero volts is the only safe voltage and the nearer to each other we can keep all our body parts the safer we will be.

So two distinct things Earthing and Bonding .

However.
Fortuitously, bonding will often assist to bring down the effective earth path impedance to a smaller figure and therefore cause a fuse/breaker etc to operate which can be helpful sometimes.

*****************
Where ever you go on this planet you will always find any two places that you casn touch will be at different electrical potentials (voltages).
Just in the same way that wherever you go on this planet you will always meet someone whose Grandad was born in Knotty Ash!