Irish and British Standards

Hi Dave,

A standard (whether BS or Irish) is only (legally) applicable if called up in a contract specification or in legislation. You will therefore need to find out what is specified for the particular case in question.

Thanks Alasdair, subsequent to this posting I received a response from BASEC stating that ‘Ireland recognise and adopt the BS 8436 Std. but Britain does not adopt the Irish Std. IS 273’.   Is this particular instance where the cable Manufacturer states the cable is both IS 273 and BS 8436 I suppose we need to discard the Irish Std. which states the cable can withstand a higher fault current of 200A and go with BS 8436 which states it can withstand 160A fault current to the cable screen.

These figures relate to a nail penetration test, and subtleties of what is assumed in  the test method.

Of course (like the cores for that matter) the jacket cannot withstand 200A or even 160A continuously, but equally it could take  a good deal more for a very short duration, potentially a few kA for less than a millisecond without burning back from the ‘nail’.

Rather these figures relate to the fast tripping of the slowest MCB, and highest likely external PSSC, to give the highest  let-through energy.

In reality a breaker does not alter the magnitude of the fault current,  rather it alters how long it flows for, hopefully by shutting it off PDQ ?.

The let through energy of an MCB depends on the external PSSC, as unlike a fuse, although it speeds up with higher fault current to a degree, beyond a certain point, the moving contacts will not go any faster.

In any case if you are wiring up in the UK and the earthing is TNC-s or TNS, use the BS figures, and all will be well. In practice this 160A limits you to a B32 or a C16 breaker.

Obviously on TT it is the RCD tripping time that would matter, and the fault current will not really come into it, as Zs will limit the current.

mike.