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Clarification of Table A444.2

Ben Gregory

Hello folks,

Thinking about BS guidance for separation distances for EMC. 

BS EN 50174-2:2018 gives a nice formula for working out distances between power supplies /control cable, with criteria to be met. 

BS 7671 Table A444.1 gives some minimum values which I'm Ok with, as I can arrive at the same values using the criteria and formula from BS EN 50174-2:2018.

however BS7671 goes on to state that if the criteria for Table A444.1 cannot be met then Table A444.2 should be used. I find Table A444.2 more problematic as I cannot understand where the values were derived from, and it means I can't use the formula in 50147 for anything over 600A , or IT application unknown, when actually 50174 allows for both those scenarios. 

What is the source material for Table 444.2?

Why does it use obsolete nominal voltages 240 and 415V?

Why does it consider voltages (the left side of the table) for separation when EMC is current driven?

Is the 600A mentioned three phase or single phase?

Many thanks for anyone taking the time to humour me and my ruminations.

Ben

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  • 0

    I'll try again.

    Ben Gregory said:
    What is the source material for Table 444.2?

    In BS 7671:2008+A3:2015, CIBSE AM7 IT and Buildings was referenced. Reference to AM7 was removed for BS 7671:2018 as AM7 was no longer available, but AM7 in turn got the tables (9.1 and 9.2) from an older IEE report, so the tables from the IEE report Electromagnetic Interference Report of a Public Affairs Board study group (1987) - were included instead of the reference to AM7.

    Ben Gregory said:
    Why does it use obsolete nominal voltages 240 and 415V?

    Quite simply, direct quote of rule of thumb from a report in 1987. I guess changing to '230' and '400' would be possible, but all the same makes no difference to the application in reality.

    Ben Gregory said:

    Why does it consider voltages (the left side of the table) for separation when EMC is current driven?

    Is the 600A mentioned three phase or single phase?

    I think Mike has covered both of those - it doesn't matter whether the 600 A is single-phase or three-phase, it's the current travelling down the conductor that's important. Differential cancellation happens less with larger csa cables (larger currents), and provided conductors are arranged in appropriate arrangement (such as trefoil, or touching for single-phase or TPN) it really makes no difference for SP vs TP.

    In terms of the voltage - electric field coupling can induce current via capacitance, I=C.dv/dt

Reply
  • 0

    I'll try again.

    Ben Gregory said:
    What is the source material for Table 444.2?

    In BS 7671:2008+A3:2015, CIBSE AM7 IT and Buildings was referenced. Reference to AM7 was removed for BS 7671:2018 as AM7 was no longer available, but AM7 in turn got the tables (9.1 and 9.2) from an older IEE report, so the tables from the IEE report Electromagnetic Interference Report of a Public Affairs Board study group (1987) - were included instead of the reference to AM7.

    Ben Gregory said:
    Why does it use obsolete nominal voltages 240 and 415V?

    Quite simply, direct quote of rule of thumb from a report in 1987. I guess changing to '230' and '400' would be possible, but all the same makes no difference to the application in reality.

    Ben Gregory said:

    Why does it consider voltages (the left side of the table) for separation when EMC is current driven?

    Is the 600A mentioned three phase or single phase?

    I think Mike has covered both of those - it doesn't matter whether the 600 A is single-phase or three-phase, it's the current travelling down the conductor that's important. Differential cancellation happens less with larger csa cables (larger currents), and provided conductors are arranged in appropriate arrangement (such as trefoil, or touching for single-phase or TPN) it really makes no difference for SP vs TP.

    In terms of the voltage - electric field coupling can induce current via capacitance, I=C.dv/dt

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