R
rocker
If you extend meter tails more than 3m you need a switch fuse, right? Would you be able to use, say, a 100A DP MCB in an enclosure instead? If not, why not? Ta.
Discuss Using MCBs instead of switch fuses on tail extensions???? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net
If you extend meter tails more than 3m you need a switch fuse, right? Would you be able to use, say, a 100A DP MCB in an enclosure instead? If not, why not? Ta.
BS 88 fuses have high breaking capacities. For example, fuses to BS 88-2.2: 1988 with a rated voltage not exceeding 660 V a.c. have a minimum breaking capacity of 80 kA, and 40 kA where their rated voltage does not exceed 500 V d.c.
There are two basic types of BS 1361 fuse: Type I, which has a breaking capacity of 16.5 kA at a lagging power factor of 0.3 and a voltage not exceeding 110 % of rated voltage and Type II which has a breaking capacity of 33 kA at a lagging power factor of 0.3 and a voltage not exceeding 110 % of rated voltage.
Circuit-breakers to BS EN 60898 have rated ultimate short-circuit capacitiesof 1.5, 3.0, 4.5, 6.0, 10, 15, 20 and 25 kA.
Maybe this is why a circuit breaker of 100A should not be used on the main tails.
Regards
Dichroic
The above info is accurate, however the CB proposed will be on the load side of the meter and main fuse which will likely have the characteristics you have mentioned
The CB is ony for protecting the extended tails which are part of the installation, the PFC issue is still covered by the main fuse and discrimination is achieved where a B or C breaker is used
Trebor
But a circuit breaker would, right? I reckon the switchfuse idea comes from a time when MCBs were expensive, but now they are much cheaper than switchfuses..... I can't think of any reason why not, but I'm sure there is one.
So if there was a fault on the tails at say 16KA and the breaker is rated at 10KA are you now saying that we let the breaker go into meltdown why the 1361 fuse is rupturing ?
The BSEN60898 mcb's in a consumer unit are usually rated at 6KA. These are also on the load side of the meter so are we relying on the main fuse to stop these committing suicide in the event of a fault of negligible impedance ? If so why do we have to test PSCC and PFC to ensure the mcb's are rated correctly?
Regards
Dichroic
Note. For overcurrent protective devices incorporated in a consumer unit complying with BS EN 60439-3: 1991 including Annex ZA, dated 2006 and having a rated current (In) not exceeding 50 A, the rated short-circuit breaking capacity may be taken as 16 000 A. This rating is however conditional upon the consumer unit being protected by a type II fuse to BS 1361 rated at not more than 100 A, or any other short-circuit protective device having the same let-through energy (I2t) and cut-off current.
Reply to Using MCBs instead of switch fuses on tail extensions???? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net
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