Minimum values of insulation resistance

I don't agree. What about installations with multiple levels of distribution?


For example, even the simple case of Main three-phase switchboard feeds a number of three-phase boards, which in turn feed some equipment plus some single-phase boards. To comply with 643.2, the minimum requirement is for each of the following to be 1 MΩ (unless of course the entire switchboard with everything connected complies):

 - The main switchboard including any final circuits it supplied

 - Each distribution circuit of the main switchboard (individually) through the DB it supplies, having all final circuits connected, along with the sub-mains this DB supplies to the single-phase DBs, along with the supplied single-phase DBs with all final circuits connected.


So, whilst all DBs will have to meet the requirement, in the case of the three-phase DBs in the tier above, their tests must include not only their submains and final circuits, but also the submains (and associated DBs and all of their final circuits) too.

Interesting - I'd read 'the main switchboard and each distribution circuit tested separately' to mean that each distribution circuit should be tested individually - whether it originates from the main switchboard or from a sub-board (the wording doesn't specify distribution circuits hailing from the main switchboard). So in each case, the test is effectively the DB together with the circuit that supplies it (whether a distribution circuit or connection to the origin) and its final circuits - but not distribution circuits it feeds as they'll be tested along with the downstream DB. That way the approach is consistent and you're testing similar-sized units regardless of how many tiers of distribution the installation has.

 

It's not exactly the plainest of plain English!

you're not wrong there!


   - Andy.

I don't agree. What about installations with multiple levels of distribution?


For example, even the simple case of Main three-phase switchboard feeds a number of three-phase boards, which in turn feed some equipment plus some single-phase boards. To comply with 643.2, the minimum requirement is for each of the following to be 1 MΩ (unless of course the entire switchboard with everything connected complies):

 - The main switchboard including any final circuits it supplied

 - Each distribution circuit of the main switchboard (individually) through the DB it supplies, having all final circuits connected, along with the sub-mains this DB supplies to the single-phase DBs, along with the supplied single-phase DBs with all final circuits connected.


So, whilst all DBs will have to meet the requirement, in the case of the three-phase DBs in the tier above, their tests must include not only their submains and final circuits, but also the submains (and associated DBs and all of their final circuits) too.

Interesting - I'd read 'the main switchboard and each distribution circuit tested separately' to mean that each distribution circuit should be tested individually - whether it originates from the main switchboard or from a sub-board (the wording doesn't specify distribution circuits hailing from the main switchboard). So in each case, the test is effectively the DB together with the circuit that supplies it (whether a distribution circuit or connection to the origin) and its final circuits - but not distribution circuits it feeds as they'll be tested along with the downstream DB. That way the approach is consistent and you're testing similar-sized units regardless of how many tiers of distribution the installation has.

 

It's not exactly the plainest of plain English!

you're not wrong there!


   - Andy.