Bs7671

[Monkeyblaine]

I seem to remember when in college a rule for when using the figures within the BS7671. It was something like divide by 0.8 rule of thumb to certain figures to allow for temperature considerations. As figures in bs7671 and the on-site guide differ slightly. Can anyone elaborate on this and explain it to me please?

 

[amo]

on table 41.3 they give you the uncorrected (tabulated) figures. to correction factor, you have to mutliply the bs7671 figure by 0.8. this is the max Zs allowed on the circuit.

eg type B 32amp bs7671 = 1.44 ohms

final with correction factor - 1.44 x 0.8 = 1.152 ohms

 

[telectrix]

the osg figures are the BS7671 values x 0.8. i dispute the osg values for most applications as when does a cable ever run at 70deg?

 

[amo]

I suppose its for the worst case senario. Excess load on the cable so the protective devices would still operate??

 

[Monkeyblaine]

Excellent thanks. Is it just for max zs values?

 

[telectrix]

yes. your measured values should be below the tabulated max. values, unless it's TT earthing with RCDs used for fault protection. then the max. is 1667 (for 30mA RCDs)

 

[Monkeyblaine]

Are there any other little things like that when designing from the bs 7671?

 

[telectrix]

there's 450+ pages of little things like that in BS7671, but not all are relevant to the job you're doing.

 

[Monkeyblaine]

Haha point taken.

 

[telectrix]

like playing all the right notes, but not necessarily in the right order (eric morecombe)