earthing conductor sizes

[SparkyTris]

please can I get a bit of help in reading table 4.4(i) in the OSG.

Is the idea that you simply size the earthing conductor relative to the line/neutral conductors?

I dont know how else you would do it but wanted to make sure my interpretation of the table is correct.

 

[Taylortwocities]

It is also relative to the supply type. Have a look at this, it may help

Protective Bonding Conductor Sizes

Sizes of main Bonding Conductors… This can be confusing as the size of the main bonding conductor differs depending on the type of supply. In a TN-S or TT system the main bonding conductor is…

the-regs.co.uk

 

[EFLImpudence]

please can I get a bit of help in reading table 4.4(i) in the OSG.

It would be better if you read the actual regulations.

Is the idea that you simply size the earthing conductor relative to the line/neutral conductors?

Sort of, but the required size; not necessarily that which has been fitted.

542.3 Earthing conductors
542.3.1 Every earthing conductor shall comply with Section 543 and, where PME conditions apply, shall
meet the requirements of Regulation 544.1.1 for the cross-sectional area of a main protective bonding conductor.
In addition, where buried in the ground, the earthing conductor shall have a cross-sectional area not less than that
stated in Table 54.1. For a tape or strip conductor, the thickness shall be such as to withstand mechanical damage
and corrosion.

Or, similar to the O.S.G.:

 

[SparkyTris]

Thanks. more questions though! what are I, t and k? (I'm guessing basically I is some nominal current value, and k is some resistive coefficient of the conductor... t is ... a time period?)
but in broad practical terms, the idea of these ready-reckoner tables are that you choose the earth CSA based on the size of the working conductors

 

[EFLImpudence]

Thanks. more questions though! what are I, t and k? (I'm guessing basically I is some nominal current value, and k is some resistive coefficient of the conductor... t is ... a time period?)
but in broad practical terms, the idea of these ready-reckoner tables are that you choose the earth CSA based on the size of the working conductors

Yes, I is the prospective fault current,
t is disconnection time in seconds.

usually 115 in domestic situations.

 

[EFLImpudence]

but in broad practical terms, the idea of these ready-reckoner tables are that you choose the earth CSA based on the size of the working conductors

Yes, nobody really cares - just fit the largest and it is bound to be alright.

However, if the working conductors are twice as big as they need to be, then the earthing conductor doesn't have to be as well.

 

[Taylortwocities]

what are I, t and k?

That's where you need the regs. The OSG is just a general guide for the more common installations. You need the bible if you really want to get your hands dirty.
If you'd like a great explaination, then have a look at John Ward's video on the subject. Earthing conductor sizing starts at 12:22.

 

[SparkyTris]

Yes, nobody really cares - just fit the largest and it is bound to be alright.

However, if the working conductors are twice as big as they need to be, then the earthing conductor doesn't have to be as well.

well quite, I was going to say that it begs the question, how did you size the working conductors....

 

[EFLImpudence]

well quite, I was going to say that it begs the question, how did you size the working conductors....

The same as any other conductor. If the main fuse is 60A then it doesn't need 25mm² tails.

 

[JohnW2]

Yes, I is the prospective fault current, .... t is disconnection time in seconds.

All true but, in practice, that doesn't help very much.

Whilst "I" is straightforward enough, one does not usually know what the disconnection time will be at that current (since the published t/I curves are essentially vertical at a current equal to the MCB's magnetic trip threshold {e.g. 5 x In for a B-curve MCB}) - so, in practice, if one wishes to undertake an adiabatic calculation (per 543.1.3) one has to determine the value of I²t at the relevant value of "!" from graphs of I²t vs. I provided by the manufacturer of the MCB (graphs which are not available in BS7671 or any associated documents).

Kind Regards, John
Edit: Clarified to correct error)

 

[EFLImpudence]

Did you mean 'In' ?

 

[JohnW2]

Did you mean 'In' ?

No, I obviously didn't. I meant "N x In" - where N would be, for example, 5 for a B-curve MCB. Thanks for noticing - I will apply a clarifying correction to my post !

Kind Regards, John

 

[EFLImpudence]

Ok - not sure I understand; isn't that 'Ia' ?

Anyway, surely that would result in a tiny CPC csa and is not really the value used for the adiabatic equation. That is more the fault current calculated from the Zs figure which would be much greater.


For the Earthing Conductor, the OPD would be the main incoming fuse.

 

[JohnW2]

Ok - not sure I understand; isn't that 'Ia' ?

I've never really understood what "Ia" (alone) represents, since it only seems to have meaning when viewed in conjunction with some specified disconnection time - since it is defined as "the current causing operation of the protective device within the specified time".

Anyway, surely that would result in a tiny CPC csa and is not really the value used for the adiabatic equation. That is more the fault current calculated from the Zs figure which would be much greater.

Yes, of course, but I think you're overthinking the statement I made. I was merely pointing out that t/I curves are essentially vertical at the ('worst case') magnetic trip threshold for an MCB - and, as you say, a fault current could be much higher than that. That's why, as I said, one cannot determine the disconnection time (hence I²t, for an adiabatic calculation) at the PFC from the standard t/I curves. Much the same is true of fuses, even though the curves never become 'vertical' - indeed, the curves for fuses (as well as MCBs) in BS7671 all stop at 100 ms, so one cannot determine a disconnection time less than that.

Are you suggesting that, contrary to what I said, you have some way of determining I²t for an diabatic calculation without access to I²t/I curves for the relevant OPD (which curves, as I said, do not exist in BS7671 or any associated documents)?

For the Earthing Conductor, the OPD would be the main incoming fuse.

That's true if you feel the need to consider the (very improbable) possibility of an L-E fault between DNO fuse and CU. If you were prepared to ignore that remote possibility, then the relevant OPD would presumably be the highest-rated of the MCBs in the CUI?

Kind Regards, John

 

[plugwash]

That's true if you feel the need to consider the (very improbable) possibility of an L-E fault between DNO fuse and CU.

I think a fault inside the CU but before any protective device is probablly more likely than a fault between meter and CU. Particularly with a metal cased CU.