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Current carrying capacity of XLPE/SWA cables

M Gilmore
Hi, I keep coming across the same issue in regards to the current carrying capacity of a XLPE/SWA cable. I always use the de-rated current carrying capacity when I am not sure if all associated equipment is rated at  90°c, which is all the time. So as an example, a 4 core 150.0mm² XLPE/SWA cable clipped direct is rated at 386 Amps from table 4E4A column 3 in BS7671:2018. A standard 4 core 150.0mm²  PVC/SWA is rated at 306 Amps from table 4D4A column 3. This is a big difference! If this was protected by a 355 Amp BS 88 I would flag the conductor as being undersized.


There is a note by table 4E4A that points out Regulation 512.1.5. This is regarding compatibility and says equipment should not be connected to conductors intended to operate at a temperature exceeding 70°c unless the equipment manufacturer has confirmed that the equipment is suitable for such conditions. 99% of the time I wouldn't have the available data on site. Most circuit breakers I have checked are rated at 70°c.


Any thoughts? I just want to make sure I'm providing the correct information. I don't want to provide an observation if the conductor is ok to be rated at full capacity. Cheers in advance.


  • 0

    OMS:

    Take a look at table 6 in BS EN 61439 - that should tell you that the allowable temperature rise on terminals is 70K (usually the ambient is 20C) - so compliant switchgear could easily be operating at 90C terminal temperature when at full load - which will clearly melt PVC.



    That's helpful. So if the manufacturers' data are not available, get a copy of the relevant standard.

    For the vast majority of cases the system is neither fully loaded nor constantly loaded - but it is a common mistake made by designers and then picked up by testers - where it becomes a shitstorm of acrimony and argument based entirely on "What Iffery" that has no bearing on reality.

    In other words, allow for diversity. I think that the OP was concerned (as I would be) that the difficulty that many (most?) testers would have is verifying the design assumptions and calculations if the records are absent.


    It may be relatively easy to get to grips with the design assumptions. Let's take the example of a small joinery business, initially with a sole trader who has half a dozen machines plus chip extraction. Clearly working hours are limited, only one machine will be in use at any one time, use is intermittent, motors go on and off load as planks are fed in and spat out again, etc. It may be necessary to allow for future developments - e.g. the joiner takes on an assistant.


    It may well be possible to plug all the assumptions into the right software, but at the lighter end of the scale, it is easy to see why testers (and designers) find it easiest to ignore diversity.
  • 0

    OMS:

    Take a look at table 6 in BS EN 61439 - that should tell you that the allowable temperature rise on terminals is 70K (usually the ambient is 20C) - so compliant switchgear could easily be operating at 90C terminal temperature when at full load - which will clearly melt PVC.


    XLPE insulated copper connected to compliant switchgear can happily run at 90C - you would expect the external surface temperature of the cable to be around 80C


     




    Have you asked many switch gear manufacturer's if they would warrant such a situation?  How many would?  


    My experience is that perhaps only Schneider, on certain devices, might concede to allow conductor temperatures rise to 90C. 


    As far as a nay saying a"what if" argument, it is understandable that the conductor once arriving at the terminal would have had some unspecified opportunity to lose some heat.   However, as far as commercial risk goes, if the design (and therefore the designer) were already under some sort of scrutiny, it would be easy for someone to point to "incompatible" temperatures as a deficiency, unless of course you can prove by calculation that the conductor temperature will not significantly riser above 70C or that the switch gear manufacturer agreed that a rise to 90C is acceptable..

  • Former Community Member
    0 Former Community Member

    Dutch of the Elm:


     


    OMS:

    Take a look at table 6 in BS EN 61439 - that should tell you that the allowable temperature rise on terminals is 70K (usually the ambient is 20C) - so compliant switchgear could easily be operating at 90C terminal temperature when at full load - which will clearly melt PVC.


    XLPE insulated copper connected to compliant switchgear can happily run at 90C - you would expect the external surface temperature of the cable to be around 80C


     




    Have you asked many switch gear manufacturer's if they would warrant such a situation?  How many would?  
    Pretty well any manufacturer providing BS EN 61439 compliant switchgear - although I think we are at cross purposes - I was pointing out that the switchboard terminals at full load may already be at 90C - and that impacts the cabling. I think you are suggesting the opposite ie the cable is at 90C and influencing the terminals

    My experience is that perhaps only Schneider, on certain devices, might concede to allow conductor temperatures rise to 90C. 
    Have a read through the standard - temperature rise testing is now an integral part of the standard - many manufacturers struggle to comply with the requirements (and thus limit the cable temp to 70C, to give the switchgear some headroom

    As far as a nay saying a"what if" argument, it is understandable that the conductor once arriving at the terminal would have had some unspecified opportunity to lose some heat.   However, as far as commercial risk goes, if the design (and therefore the designer) were already under some sort of scrutiny, it would be easy for someone to point to "incompatible" temperatures as a deficiency, unless of course you can prove by calculation that the conductor temperature will not significantly riser above 70C or that the switch gear manufacturer agreed that a rise to 90C is acceptable..

    That temperature rise is specified in the switchgear standards - this should provide data on the terminal temperature and enclosure temperature - I agree that some manufacturers may limit conductor temperature to 70C to allow the heat input into the switchboard to not exceed the limits of the switchgear standard - but certainly not all


     




    Regards


    OMS

  • 0

    ...Pretty well any manufacturer providing BS EN 61439 compliant switchgear - although I think we are at cross purposes - I was pointing out that the switchboard terminals at full load may already be at 90C - and that impacts the cabling. I think you are suggesting the opposite ie the cable is at 90C and influencing the terminals


    ... many manufacturers struggle to comply with the requirements (and thus limit the cable temp to 70C, to give the switchgear some headroom




    I had not considered it from that angle, i.e. the extra heat-transfer to the switchgear might actually push the switchgear over 90C, rather than just it could not safely operate between 70C and 90C.  Albeit this does still leave us in the situation where we should assume 70C unless we can confirm that  higher conductor operating temperatures will be sustainable.

     

    Thanks OMS.

  • Former Community Member
    0 Former Community Member
    OK - perhaps this will help:

    Built-in components 

     

    In accordance with the relevant product standard requirements for the individual components or, in accordance with the component manufacturer’s instructions , taking into consideration the temperature in the assembly.


    So if the external insulated cabling connects directly to (in this case) the circuit breaker output terminals the switchboard manufacturer states the limit of the connected conductor (often 70C as directed by the circuit breaker supplier) - this would be in COTS switchgear with minimal form of separation


    However:

    Terminals for external insulated conductors  - 70K


    This would be the case in say a Form 4 Type 6 Switchboard, where the circuit breaker output terminals are extended by the switchgear manufacturer to the glanding/termination enclosure for connection to the external insulated enclosure. In this case, the temperature rise on the terminals could be 70C (from a 20C ambient) so would put at risk the 70C cable insulation, but would allow you to make full use of the 90C capability of XLPE


    You can play safe and use XLPE insulated cabling constrained in operation to 70C for the design current and that way, you can comply (probably) with no further knowledge of the switchboard design criteria - but you don't have to if you know what the switchboard is designed to do


    Regards


    OMS




  • 0
    Thanks OMS.  That is helpful.


    May I ask a tenuously-linked, probably slightly off topic question? 


    What property most dictates the thickness of the insulation on conductors in typical cables, dielectric, thermal, or mechanical?  My question comes from considering the mode of failure for cables where they have been severely pinched.  Some might suggest that the reduction in dielectric strength due to reduced insulation-thickness would allow some current-leakage, and eventually a breakdown of insulation through the associated heat-effects.. Me... I'm not sure.  


    Sorry for the hijack attempt.
  • 0
    in general installation, the concern is mechanical. The break down voltage of undamaged insulation is far higher than the working voltage.

    If you want a feel for what minimum insulation thickness could be if robustness was not a concern, then look at the thin varnish-like insulation on wire used for motor and transformer windings.

    At higher voltages (many kV) then the insulation needs to be re-inforced accordingly. Even so at DC a cm of polythene will easily hold off 150kV,  but no cable maker would dare sell that as a working voltage for genera installation.

      (you need to be  more careful than a simple breakdown figure for AC because of displacement current causing corrosive corona at the air to insulation interface, so there is an additional bend radius to consider. There are tricks such as grading insulation as a series of cylinders with equipotential metal foils in between to force a regular voltage profile and smooth over any bubbles. Corona dicharge in any defect or  bubble in the insulation will lead to early failure.)
  • 0

    ... if robustness was not a concern, then look at the thin varnish-like insulation on wire used for motor and transformer windings.

     




    Good point. 


    Doing my best to not continue the thread-hijack too far... but..


    my question was brought to mind when reading one of the [factory] test methods for AFDDs.  There are few tests but the gist of them is to cause damage to a sample cable, connect it to the subject AFDD and prove the AFDD picks up any arcing.  One test uses 7kV+ to "condition" an already damaged sample, to create an arc-track between cores... and in theory an arcing fault for the AFDD to detect. 


    I obviously queried to myself whether the tests are at all realistic and wondered if the one I mention tries to mimic a pinched cable that later via some current flow between cores (L-N) develops a fault that then becomes partially carbonised...


    Based on the dielectric strength of even thin insulators, the above looks doubtful.


    Thanks,

  • 0
    The fixed wiring of a 230/400V installation up to and including the sockets is supposed to be able to withstand 4kV transients.
  • 0

    wallywombat:

    The fixed wiring of a 230/400V installation up to and including the sockets is supposed to be able to withstand 4kV transients.




    Interesting.  I can see why that would be expected of cables. Is that a requirement of BS 7671 or product standards?  


    Thanks,

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