Main Bonding in TT (and TN-S) installation

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In a recent thread ...

So what EICR code would you give to undersized bonding?, personally I'd probably look towards a C3 for the 6mm main bonding on a domestic TNCS (remember though that if TT, it might not be undersized at all) ....
This comment from @Adam_151 reminding me that I have never really been sure about the thinking behind requirements for main bonding conductors in a TT (or, come to that, TN-S) installation, the situation seemingly being that the 6mm² minimum for such conductors is essentially arbitrary (and electrically 'OTT'). However, my thinking may be flawed!

Unlike the situation with TN-C-S, in a TT (or TN-S) installation there is no risk that, under certain fault conditions, the combined neutral current of several installations might try to travel through one's main bonding conductor. Accordingly, the only situation in which high currents might flow through the bonding conductor is as a result of an "L-E" fault within the installation, and even that only if the bonded extraneous-c-p represents a low resistance/impedance path to true earth.

Of course, if (as 'required') everything in a TT installation is provided with fault protection with RCDs, then an RCD should clear any L-E fault very rapidly, so this discussion becomes essentially moot. The same is essentially true of a TN-S installation, provided that all circuits have (as 'required') adequate fault protection (from OPDs and/or RCDs).

When there is an "L-E" fault, the fault current will be shared between the installation's earth and the bonded extraneous-c-p. Assuming the bonded extraneous-c-p has a fairly low impedance (much lower than that of the TT electrode) to true earth, that means that (until the fault is cleared) in a TT installation, most of the fault current will flow through the bonding conductor, whereas in a TN-S installation the proportion going through the bonding conductor will depend on the relative impedances of the two paths to earth.

Hence, since the problems specific to TN-C-S do not exist, and the only high currents that can flow through the bonding conductor with TT or TN-S are those arising from faults within the installation, then in terms of the ability to 'cope with' those potentially high currents, the main bonding conductor is in no different a position from the CPC of the circuit with a fault.

In other words, in terms of that consideration, if the CPC of the circuit with the highest PEFC is 'adequate' (adiabatically), then there is no apparent electrical need for the main bonding conductor to have a larger CSA than that CPC - which in practice means, electrically speaking, a much smaller bonding conductor than 6mm² would invariably be 'adequate.

So, from that consideration (ability to cope with fault current) the 6mm² minimum for main bonding conductors in TT and TN-S installations seems to be essentially arbitrary, not based on 'electrical need'. Nor can I really think of any other 'considerations'. They might have been concerned about possible voltage drops along the length of the bonding conductor (possibly leading to unacceptable PDs), but since there CSA requirements do depend on conductor length, that does not seem to be part of their thinking.

Are there any flaws in my thinking? I suppose a (very improbable) L-E fault upstream of final circuits might possibly result in higher fault currents in a bonding conductor than would be possible with a fault on any of the final circuits?

Kind Regards, John
 
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Might consideration of the mechanical strength of an insulated but unsheathed bonding conductor of the minimum size required electrically indicate a need for a larger minimum size to reduce the possibility of the conductor being accidentally severed, possibly without it being noticed?
 
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Might consideration of the mechanical strength of an insulated but unsheathed bonding conductor of the minimum size required electrically indicate a need for a larger minimum size to reduce the possibility of the conductor being accidentally severed, possibly without it being noticed?
Despite EFLI's "No", I certainly considered that as one of the most likely possibilities - after all, for 'single' CPCs, they require 4mm² unless 'protected against mechanical damage' (in which case 2.5mm² is acceptable). Even more dramatic (although perhaps partially understandable), in the case of buried earthing conductors they require only 2.5 mm² if 'protected from mechanical damage', otherwise 16mm². In neither case is the required CSA 'without protection from mechanical damage' even remotely necessary, electrically.

Kind Regards, John
 
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Yes, but Dave said would "the minimum size required" have to be bigger to prevent it being severed?

The minimum size required already accounts for mechanical strength.
 
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Yes, but Dave said would "the minimum size required" have to be bigger to prevent it being severed? ... The minimum size required already accounts for mechanical strength.
I think you may be trying to take his wording over-literally. Was he not simply saying that (as you and I are saying) that the "minimum size required" takes on board considerations of liability to mechanical damage as well as electrical considerations?

If there's an inconsistency it's that they don't seem to 'offer a choice' with main bonding conductors, whereas with 'single' CPCs and buried earthing conductors (and maybe other things?) they allow a smaller CSA IF there is 'protection from mechanical damage'.

Kind Regards, John
 
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Ok, but I still think the answer is 'no'.

The minimum size allowed (under any circumstances) for a main bonding conductor is 6mm², therefore one which had an electrical requirement to be 6mm² would not have to be any larger for any other reason.
 
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Oh, I see where the mis-understanding is. What I'm trying to say is that I would think there is an absolute minimum size for a bonding conductor to ensure adequate mechanical strength regardless of the required current carrying capacity. That is, even if the required current carrying capacity could be met buy 1.5 mm squared bonding conductor that would not be mechanically strong enough therefore a larger one i.e. the minimum that I mentioned above must be used. If a larger bonding conductor is needed to carry the required current then clearly that meets the requirement for mechanical strength being greater than the minimum required for mechanical strength.
 
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Ok, but I still think the answer is 'no'. The minimum size allowed (under any circumstances) for a main bonding conductor is 6mm², therefore one which had an electrical requirement to be 6mm² would not have to be any larger for any other reason.
I think we're all agreed and are just talking at cross-purposes.

As I said in my OP, I suspect that there is no electrical reason for a bonding conductor in a TT (or TN-S) installation to be anything like as big as 6mm² - so the fact that the regs require a minimum of 6mm² suggests that that requirement results from other considerations (i.e. liability to mechanical damage) being taken into account in addition to the electrical requirements.

Kind Regards, John
 
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Mechanical damage should not happen if it complies with 543.3.1
Eh? 543.1.3 is tha adiabatic calculation which, as I said/implied in my OP, would (as far as I can see) probably usually show that (in a TT installation) a CSA considerably less than 6mm² would be adequate, in terms of electrical considerations, for a main bonding conductor.

Since, despite that, 541.1.1 imposes a minimum of 6mm², that presumably is due to some additional consideration other than the electrical one - and the issue of potential mechanical damage (which potentially could result in reduction of the effective CSA) would seem the most likely/'obvious' such consideration, wouldn't it?

Kind Regards, John
 
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543.3.1 sorry :)
Ah, right :) However, this is where the regs seem to get rather silly because that regulation ...
543.3.1 A protective conductor shall be suitably protected against mechanical and chemical deterioration and electrodynamic effects.
... appears to be totally contradicted by (my highlighting):
543.1.1 ....... If the protective conductor:
(iii) is not an integral part of a cable, or
(iv) is not formed by conduit, ducting or trunking, or
(v) is not contained in an enclosure formed by a wiring system
the cross-sectional area shall be not less than 2.5 mm² copper equivalent if protection against mechanical damage is provided, and 4 mm² copper equivalent if mechanical protection is not provided (see also Regulation 533.3.1)
... and the reference back to 543.3.1 seems to make it even more contradictory!

The only very slight difference between those two seemingly contradictory regulations is that the former says that a protective conductor must be protected against mechanical deterioration, whereas the latter says that it does not have to be protected against mechanical damage (provided CSA is ≥4 mm²). Is there a meaningful difference - i.e. is it possible to protect something from 'mechanical deterioration' whilst not protecting it from 'mechanical damage'?

Kind Regards, John
 

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