Supplementary equipotential bonding in group 2 medical locations

Stop at the 0.125 ohms.  ?


How long is your bond cable that you need to be less than this resistance ?


If we had a reel  of 4mm cable, then that would be  4m /16 milliohms approx  or about 0.04 ohms per metre by my rule of 16  (its not quite true, but first cut...)


(6mm cable would be 2/3  of the resistance of this..)


(watch the units - dividing something in ohms by something in ohms/m,  cancels the ohms and gives  one over inverse metres = metres on top, where you need them .... )


So if your ceiling bond is less than 125milliohms / 4 milliohms per metre   ~ 30metres long  then some off that reel of 4mm would be fine...

So without thought we could use 4mm cable for bonds up to perhaps 25m, and then think about it a bit more at greater ranges.


Any smaller cable than that would need mechanical protection, so is easier just to use 4mm.

But you could put 2.5mm in conduit for a run of perhaps 15metres or less.

If you needed to go further, or your Ia was higher than 200Amps,

then you'd need to either have multiple links in parallel, or fatter cable.


The 0.2 ohm figure assumes a lower Ia than you have chosen. I'm not sure why the book is like that.



Note also that I am being quite careless with mental maths to make the demonstration clearer,  I know that really 4*30 is 120, not 128, but then the tables suggest 4mm may be more like 5 millioms per metre, rather than exactly 4. I justify this arm waving approach by saying that  if the quick arm-wave answer is  close enough that we are looking for a few 10% it is usually worth using the next size up, or taking 30 mins and doing the sums properly, but it saves a lot of time in the majority of cases that are clearly OK  - say if that bond was only 5 or 10m long... 

Things designed to be within a cigarette paper thickness of the spec limit have no margin for later updates or minor errors in the assumptions - is your mains 230V today or 240, that sort of thing, and that can mean trouble later if the cable route is a tad longer than expected or whatever.)

 Mike.

Stop at the 0.125 ohms.  ?


How long is your bond cable that you need to be less than this resistance ?


If we had a reel  of 4mm cable, then that would be  4m /16 milliohms approx  or about 0.04 ohms per metre by my rule of 16  (its not quite true, but first cut...)


(6mm cable would be 2/3  of the resistance of this..)


(watch the units - dividing something in ohms by something in ohms/m,  cancels the ohms and gives  one over inverse metres = metres on top, where you need them .... )


So if your ceiling bond is less than 125milliohms / 4 milliohms per metre   ~ 30metres long  then some off that reel of 4mm would be fine...

So without thought we could use 4mm cable for bonds up to perhaps 25m, and then think about it a bit more at greater ranges.


Any smaller cable than that would need mechanical protection, so is easier just to use 4mm.

But you could put 2.5mm in conduit for a run of perhaps 15metres or less.

If you needed to go further, or your Ia was higher than 200Amps,

then you'd need to either have multiple links in parallel, or fatter cable.


The 0.2 ohm figure assumes a lower Ia than you have chosen. I'm not sure why the book is like that.



Note also that I am being quite careless with mental maths to make the demonstration clearer,  I know that really 4*30 is 120, not 128, but then the tables suggest 4mm may be more like 5 millioms per metre, rather than exactly 4. I justify this arm waving approach by saying that  if the quick arm-wave answer is  close enough that we are looking for a few 10% it is usually worth using the next size up, or taking 30 mins and doing the sums properly, but it saves a lot of time in the majority of cases that are clearly OK  - say if that bond was only 5 or 10m long... 

Things designed to be within a cigarette paper thickness of the spec limit have no margin for later updates or minor errors in the assumptions - is your mains 230V today or 240, that sort of thing, and that can mean trouble later if the cable route is a tad longer than expected or whatever.)

 Mike.