Standard DNO Fault Current - incompatible MCB rating

Question

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...?

broadgage · Accepted Answer

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.

AJJewsbury · Answer

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.

Chris Pearson · Answer

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!

John Peckham · Answer

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

John Peckham · Answer

Use the function in Amtech for has upstream protection which will show you what the PFC is at a particular part of the installation.

broadgage · Answer

BTW, the highest prospective fault current that I have have found at a 13 amp socket outlet. was about 20 Ka not actually in the substation, but immediately adjacent. 
 400 amp circuit from substation, into customers main switchgear, this supplied sub-mains to different areas of the building, and a small fuseboard adjacent. Socket outlet on side of fuseboard.

Standard DNO Fault Current - incompatible MCB rating

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