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.
