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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

  • 0

    Well first it is important to realise this is attempting to provide very simple rules of thumb for situations that are actually almost too complex to solve based on other standards that are equally inderterminate. Clearly actually the amplitude and frequency  within  the signal cable will affect how may mV in that cable actually cause the signal system to malfunction, as will the length of the parallel sections, and in real systems the 'safe' separation may be way closer or indeed further apart than the standards suggest. Also 'how noisy' is typical mains?  3 phase or 1 phase does not come into it - we are looking at how the wire functions as an antenna for non mains frequencies, either desired ones or not.
    The 600A limit is a bit arbitrary, but is a caution that these rules of thumb do not work with very large cables and you should calculate or measure the disturbance yourself, or more likely use an armoured cable and a larger clearance or solid trunking  and not worry too much.
    Mike.

  • 0

    I have found that in real life situations I can measure 240 Volts and 415 Volts in U.K. installations despite the declared Voltages of 230 and 400 Volts.

    Z.

  • 0

    Well, I did post a reply ... not sure where it went to ... with some detail regarding this, and answers to the questions.

    Might have gone into a moderation queue for some reason?

  • 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

  • 0
    Ben Gregory said:
    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. 

    Is it not the case that A444.4 states (1st para);

    Where both the specification of the information technology cable and its intended application is known, the requirements of BS EN 50174-2 and BS EN 50174-3 are appropriate.

    A444.4 then goes on to describe some more detail ... and then says 'If the above conditions do not apply ...' meaning the conditions where both the specification of the information technology cable and its intended application is known' then go ahead an follow the older rules of thumb if you need to.

    So I read it as though if you can use BS EN 50174-2 or BS EN 50174-3, then do so, otherwise, use the rules of thumb.

    More pertinent, however:

    It is wholly incorrect to say 'it means I can't use the formula in 50174 for anything over 600 A', because the whole of Annex A444 is informative, which means it is only guidance, and not necessary to comply with BS 7671. If there are normative requirements in BS EN 50174 series for this particular aspect, it would be proper to use those; if the requirements are informative, they would be equally valid.

    However, please refer to BS 6701 Telecommunications equipment and telecommunications cabling. Specification for installation, operation and maintenance as this modifies certain aspects of BS EN 50174-2, particularly in terms of separation (segregation) of power and telecommunications cabling.

  • 0 in reply to mapj1

    Many thanks Mike

  • 0 in reply to Zoomup

    Yes indeed this is usually the case. Thanks Z.

  • 0 in reply to gkenyon

    Thanks Graham, apologies for radio silence.

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