Using MCBs instead of switch fuses on tail extensions????

[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.

 

[DurhamSparky]

2 birds one stone!!! never thought of that!! never used switch fuse either...!!

come to think of it i thought a switch fuse was used when you add additional supply from meter that is in smaller cable not suitably rated etc..

like 25mm tails on 100a fuse and you add a henly with 16mm tails in a 40a/60a switch fuse...!!

i thought a DP isolator was just needed for 3m+ runs!!!


i may be wrong....! not right every day!!

 

[SirKit Breaker]

Dont think an Isolator is acceptable because a switch fuse is there to protect the suppliers cut out fuse. An isolator on its own wont give that protection.

 

[rocker]

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.

 

[DurhamSparky]

im with you rocker...?? 100a cut out and 80a MCB DP....!!! would make better work and hell of lot easier to install and look after..!

 

[trebor]

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.

The simple answer is yes perfectly acceptable, circuit breakers are recognised as isolation devices in the regs, so you are quite right with isolation and fault protection in one, switchfuses go back to long before cb`s existed

the dno requirements are for in excess of 3 mtrs unless the extension is in earthed metalic conduit or an swa cable

 

[rocker]

Now that is very interesting. Seems like I've been wasting money left, right and centre!

Now for something specific to a job I have coming up.... Would I be able to use two DP circuit breakers in one 4-gang enclosure, properly labelled, to use as the meter tails isolation to two flats do you think? Or can anyone think of any reason they need to be physically seperate encloures? I'm just thinking on a tidyness angle now....

 

[trebor]

The dno ( im speaking as a retired board inspector now) will want seperate protection for each riser, bear in mind that these may be metered on different phases

 

[dichroic]

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 capacities

of 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

 

[Amp David]

Used one of these before on the tails

http://www.tlc-direct.co.uk/Images/Products/size_3/CPD01P.JPG

along with a 63amp MCB.

 

[trebor]

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 capacities

of 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

 

[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

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

 

[SirKit Breaker]

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.

Sorry mate I am a knob didnt read the question properly

 

[Des 56]

As far as I am aware
In a domestic installation,the suppliers 1361 fuse is considered adequate protection against prospective fault current, allthough the downstream devices ratings are lower
This is because that fault current for a 230 volt supply up to 100 amp will not exceed 16 KA

Regulation 536
and
page 56 and 57 of the on site guide among other things will suggest this is so



This is someone elses take on the subject(not mine)

Because of their current limiting capability fuses can provide, when connected in series, back-up protection to other overcurrent protective devices, for example circuit breakers.
With the correct co-ordination with the fuse the limitation of let through energy can allow such devices to be used in applications where the short-circuit level exceeds the device rating.

 

[dichroic]

As far as i can tell;

Regulation 434.5.1 requires the rated short-circuit breaking capacity of each device for protection against fault current to be not less than the prospective short-circuit current or earth fault current (whichever is the greater) at the point at which the device is installed.

A lower breaking capacity rating is permitted where so-called ‘back-up’ protection is provided. Back-up protection is where another protective device (or devices), having the necessary rated short-circuit breaking capacity, is installed on the supply side, and the characteristics of the protective devices are co-ordinated. The co-ordination is required to be such that the energy let-through (I2t) of the device or devices does not exceed that which can be withstood, without damage, by the device or devices on the load side.

Regards

Dichroic

 

[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.

 

[trebor]

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

I admit that the regs as usual are a bit of a mish mash with lots of ifs buts and maybe`s but

In the extreme case of very high pfc where breakers are used at origin both b and c curve mccb`s are available at 10ka and above, this is normally only the case with industrial services of 200A and above where the service cables are larger and could be the case where a large service is brought into a block of flats say.

The normal domestic service is unlikely to exceed 6ka

in the case of 60898 cb`s in consumer units these can be taken to withstand 16ka where they are max 50A rated, enclosed in a BSEN 60439-3 or BS5486-13 enclosure, the main fuse is Type 2 BS1361 rated at no more than 100A

Trebor

 

[trebor]

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.

Sorry Dichroic

I posted the reply below before I saw yours above, and ive just really repeated what you had already correctly said, i hope your post was not copyrighted

 

[Spudnik]

Sorry but i havent read the whole thread yet, but, MCB's generally have a 6or 10kA rating which could be blown to pieces in the event of a fault.