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Is DNO 400A BS88 ISU to 400A MCCB Selectivity Mandatory

Steven Dwight

My question is selectivity between local DNO 400A BS88 ISU to 400A MCCB Selectivity mandatory. Whilst I feel it is always best practice; in this rare occasion we cannot achieve full selectivity and as this is a DfE project I am concerned there will be more of an issue made regards this than necessary.

The supply arrangement serving a local primary school has changed. Originally the 400A ISU was terminating adjacent the new MCCB panel within a dedicated LV switch room, located within the new building. Now a remote GRP (150m away from the electrical switch room) is being provided in the north car park, therefore introducing an additional level / tier of protection in the system.

The proposed outgoing way of the new GRP MCCB panel serving the school is a Schneider 400A NSX Micrologic 6 MCCB (this has been selected to assist with downstream selectivity). I’ve managed to achieve selectivity throughout the system (only just), though the Amtech model is evidently showing a fault between the DNO 400A ISU fuses and the 400A NSX Micrologic 6 MCCB.

(1 error – the energy let-through 803.546x103 A2s of the protective device is greater than the pre-arc energy 640x103 A2s for the upstream fuse in the DNO 400A ISU).

If total selectivity with the upstream DNO fuse is not mandatory and partial selectivity exists, and the risk of simultaneous operation is limited to a rare high-level fault at the DNO intake, can the above be discharged via a design note (or is the design note even required)? My understanding is such a fault would most likely arise from equipment failure, insulation breakdown, contamination or accidental damage during installation or maintenance; which all are low risk as only trained operators / engineers should have access to the GRP enclosure.

As always, I look forward to hearing everyone’s thoughts on this.

  • +1

    BS 7671 certainly has no blanket demand for selectivity ... indeed there are several situations where BS 7671 remand things that makes it impossible (cascaded 30mA RCDs in some special locations.installations come to mind). It can be required in certain circumstances - where there is a need for continuity of service (perhaps for safety services or economically important loads) .. often that boils down to what the customer requirements are (i.e. what they're prepared to pay for vs what losses/inconveniences they're tolerably happy to put up with).

      - Andy.

  • +1

    What are you trying to achieve?

    The only benefit of achieving selectivity in your case, where there are two similarly rated devices is series, is around time to repair.  If the protective device that operates is within your control then it can potentially be reset more quickly than one that isn't.  What would the repair time be if the MCCB tripped?  How much longer would the overall repair time be if the DNO fuse blew instead?

  • 0 in reply to DShutt

    Ideally the client 400A MCCB will operate before the DNO 400A BS88.

    On site maintenance engineers will be in control of resetting the MCCB therefore down time would be less (once the problem had been investigated). Waiting for the DNO engineers to be called to site to rectify their ruptured fuses could mean the school being without power for longer than 24hrs?

  • +1 in reply to Steven Dwight

    Since you have selectivity between the 400A MCCB and those devices downstream of it, then the only time the MCCB or DNO fuses should open is when there's a fault on the main cable itself ... in which case time to locate the fault and effect a repair is likely to be significant - the DNO fuses could probably be replaced while waiting for that to happen (the faulty section still being isolatable by the MCCB).

       - Andy.

  • +1

    The issue I can see with this, is not an issue with conforming to BS 7671. It's DNO requirements ... specifically, whether the DNO has agreed to their protective device possibly operating because there is a fault in wiring that should be covered by a consumer's protective device.

    This may lead to additional cost if the DNO fuses did operate ... or even a refusal to reconnect if there were a fault. There could also be financial penalties if there is a connection agreement in place.

  • 0 in reply to gkenyon

    Hi Graham, thanks for the response.

    We have not engaged with the DNO to discuss this, possiby ths is something we should do. If they require customers to ensure selectivity, whilst offering a total agreed capacity of 400A, they should be responsible for providing upstream cut-out protection that will assist, not limit, coordination, selectivity & full capacity.

    Prior to introduction of the remote GRP enclosure, the DNO were to enter the main builidng electrical switch room via their standard local mounted ISU & metering arrangement. We were providing a point of isolation only -  (lockable TP&N main switch disconnector immediately downstream of the electricity meter).

    The reason we have introduced the 400A MCCB is to offer protection of the now client owned 150m submain cable, due to the supply terminating in the remote GRP rather than direct into the building.

    The only way I see of removing the error within the calculation software: -

    • installing a switch disconnector within the GRP
    • provide a ‘non intelligent’ MCCB frame switch disconnector to serve the 150m submain
    • install a suitably rated incoming isolator within the main building switch panel
    • ensure all cabling from the DNO supply terminal to the main builidng switch panel incoming isolator are sized based on the 400A BS88 protection 

    In this scenario, the outgoing MCCBs within the main building switch panel (typically 160A) will provide total selectivity with the upstream DNO fuses; though we would be potentially be up sizing the 150m submain due to the 400A BS88 and be ultimately relying on the same fuses to be the cable protection. Which I believe the DNO would not agree with either.

  • +1 in reply to Steven Dwight
    Steven Dwight said:
    The reason we have introduced the 400A MCCB is to offer protection of the now client owned 150m submain cable, due to the supply terminating in the remote GRP rather than direct into the building.

    Yes, I think that's the point ... the DNO wouldn't usually agree to their cutout protecting a cable run of that length ... which means a fault on it should clear the consumer's protective device and not operate the DNO's ?

    Certainly worth looking into and coming to some agreement.

    Steven Dwight said:

    The only way I see of removing the error within the calculation software: -

    • installing a switch disconnector within the GRP
    • provide a ‘non intelligent’ MCCB frame switch disconnector to serve the 150m submain
    • install a suitably rated incoming isolator within the main building switch panel
    • ensure all cabling from the DNO supply terminal to the main builidng switch panel incoming isolator are sized based on the 400A BS88 protection 

    You would still need to check the DNO agrees that their cutouts can protect the 150 m run, if it's not in a connection agreement already,.

  • +1 in reply to gkenyon

    Hi,

    gkenyon said:
    Yes, I think that's the point ... the DNO wouldn't usually agree to their cutout protecting a cable run of that length ... which means a fault on it should clear the consumer's protective device and not operate the DNO's ?

    The second part of your sentence doesn't follow from the first.  I would agree that the DNO will almost certainly expect the customer to provide protection for the (150m in this case) cable from their equipment but it doesn't follow that a fault on that cable must not cause the DNO's protection to operate.

    To take an absurd example, the customer could provide a 1000A rated supply cable with appropriate protection for the cable and connect it to the DNO supply and that would meet the DNO's requirement for the customer to protect their cable.

    The DNO will expect that their protection arrangements are not relied for "normal" fault clearance within the customer's installation.  My unfounded suspicion is that the more "abnormal" fault scenarios, which in this case probably amount to a JCB through the supply cable or a major failure of customer switchgear, are unlikely to concern the DNO too much (providing that, of course, they really are abnormal and on average are only going to happen every 100+ years).

    Fundamentally, the DNO will want the design to be such that they are very unlikely to have to spend time replacing their fuses - I think that overall the proposed design meets that aspiration.

    I agree that a discussion with the DNO is the right thing here but if they disagree with what is currently proposed I would be pushing them quite hard to justify why as it makes it very difficult to make full use of the contracted(?) supply rating.

  • +1 in reply to DShutt
    DShutt said:
    I would agree that the DNO will almost certainly expect the customer to provide protection for the (150m in this case) cable from their equipment but it doesn't follow that a fault on that cable must not cause the DNO's protection to operate.

    I think this depends on which side of the fence you are sitting on.

    Quite simply, if the DNO cutout operates during a fault, it's providing the protection it was intended to provide (whether we like it or not). The only way to prevent this, is to design for selectivity. 

    In BS 7671 terms, I agree that there is no absolute requirement to provide selectivity, unless failure to do so 'adversely affects the safety of the installation' (Regulation 536.3) ... but I still think there's room for the DNO to argue that selectivity is necessary so that their device is not providing fault current protection (i.e. operating) when certain faults occur on the 150 m submain.

    DShutt said:
    I agree that a discussion with the DNO is the right thing here but if they disagree with what is currently proposed I would be pushing them quite hard to justify why as it makes it very difficult to make full use of the contracted(?) supply rating.

    Fair point.

  • +1 in reply to gkenyon

    but I still think there's room for the DNO to argue that selectivity is necessary so that their device is not providing fault current protection (i.e. operating) when certain faults occur on the 150 m submain.

    I would be surprised if the DNO were to insist on that. Not completely relying on their fuse is one thing, ensuring it never blows for a fault downstream of the next tier of protection is another. I suspect the alternative of increasing the ratings of the DNO fuses so they provided only fault and not overload protection would be even less to their taste.

    Even then some reliance on the DNO's fuse for downstream faults isn't that uncommon - e.g. in domestic situations where the DNO fuse provides "backup" protection for MCBs where the PFC might exceed the MCB's breaking capacity - i.e. for faults soon after the consumer unit it would be expected that the DNO fuse would blow (either instead of or as well as the MCB opening) - and all perfectly acceptable and documented in BS EN 61439-3 (annex ZB) and BS 7671 (536.4.201) - the latter of course being the yard-stick the DNOs use for acceptability of an installation to be connected to their system.

      - Andy.

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