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Standard DNO Fault Current - incompatible MCB rating

Neil Barrett

Hi , I have a question.....  A small office building requires a new distribution board local to a 100A TPN DNO cut-out however, in lieu of an existing EICR, the standard Prospective line fault current is 25kA (3 phase), 16kA (1 phase).  With the DB in close proximity to the DNO the OCPD for final circuits needs to be types ITRO of 25kA breaking capacity ...... which are not available.... How would you mitigate this risk?

Am I missing something...?

  • 0
    Neil Barrett said:
    in lieu of an existing EICR

    Not quite sure what that means. A PEFC of 16 kA implies a Ze of < 0.02 Ω. How close is the transformer?

    Are the final circuits 3-phase or single-phase?

  • 0 in reply to Chris Pearson

    This will be the line fault and not the earth fault and I would assume that the worst case line fault current that the DNO would state would be adjacent to the Transformer, so I could estimate that the cut-out is about 40 metres from the transformer and can model this in Amtech to see what fault current I might get.  The majority are single phase but there would still be a risk of a fault between final circuits of different phase which would present this issue if occurring close to the board. I just think this must be a common occurrence.... 

  • +1

    In domestic and similar premises it is often considered acceptable for the available fault current to exceed the breaking capacity of the MCBs used. Provided that backup protection is provided by a 100 amp or smaller BS88 fuse in the DNO cutout.

    In practice, under most fault conditions, the MCB will operate without blowing the cut out fuse. However in the case of the actual fault current being very high, then the cut out fuse may operate. A very few meters of sub-circuit cable will limit the fault current to within the capacity of the MCBs. The problem only arises if the fault is close to the consumer unit.

  • 0 in reply to Neil Barrett

    This seems to be a bit muddled, but if your service cable is 25 mm² copper equivalent (plenty for 100 A/phase), that is about 0.06 Ω for a start.

    If there are 3-phase final circuits, then your PFC is approximately twice your PEFC for line/line faults. If the supply is PME, the L/E and L/N faults at the DB will be more or less the same.

    From the sound of it, 10 kA MCBs will be fine.

  • +1

    Am I missing something...?

    Yep - backup protection - as broadgage says - see reg 536.4.201 - just about any UK consumer unit will be rated at 16kA overall regardless of the breaking capacity of individual MCBs - courtesy of annexe ZB of BS EN 61439-3.

    For other types of distribution board the same principle of using an upstream device for backup protection can also apply - see 2nd paragraph of 434.5.1 - usually DB manufacturers provide data about the rating and types of suitable upstream devices.

       - Andy.

  • 0 in reply to AJJewsbury

    Thanks Andy, 434.5.1 answers it perfectly! I was including a fuse switch disconnector with BS-88 fuses upstream of the DB which could have given me selectivity issues with the CUT-OUT fuses however I'll put an electronic 36kA MCCB instead and that will resolve my problem.

    Thanks all for your input!

  • 0 in reply to Chris Pearson

    Thanks Chris, understood. 

    I was applying the DNO stated worst case characteristics

    SSE

    "Nominal Voltage U/Uo = 400/230v
    Nominal frequency f = 50 Hz
    Prospective fault current Ipt: 25kA (3 phase), 16kA (1 phase)
    External loop impedance Ze:
    0.35 ohms typical maximum for TNCS system
    0.8 ohms typical maximum for TNS system
    The Ze values may be higher on older networks and should be verified by measurement."

  • +1 in reply to Neil Barrett
    Neil Barrett said:
    I was including a fuse switch disconnector with BS-88 fuses upstream of the DB which could have given me selectivity issues with the CUT-OUT fuses however I'll put an electronic 36kA MCCB instead and that will resolve my problem.

    If you blow either one or two 100 A fuses, I think that selectivity is the least of your problems!

  • +1

    Neal

    If you have Amtech then model the installation from the transformer terminals using the transformer data. The layers of circuit protection will have a current limiting effect along with the cable CSA and lengths. Then look at the actual fault current at the DB. 

    JP

  • 0 in reply to John Peckham

    Thanks John, good idea..

    What I have done is set the worst case DNO supply characteristics as being at the Transformer and then modelled the cable to the distribution board... Line fault current now drops below the 16kA MCB rating.. 

    This identifies to me that it is highly unlikely that there will be a line fault current of 25kA magnitude unless installing a distribution board in a sub-station!!  Therefore is rarely an issue. But I'm pleased to have gone through the process and come to this conclusion.

    Thanks all for your help.

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