Question on MCB Loading

[self builder 123]

Flameport

Just to clarify the 40A MCB in the house consumer unit which feeds the workshop is a Type B MCB and not a Type C.

 

[spark1]

You mention you have a water heater. if it is supplied via metallic pipework then you will also have a big issue regarding the equipotential zone And Exporting of the Earthing system from the Main building

 

[securespark]

Sparkyspike - 1.15 Ohms on a type B? That'd be a 32A, surely?

But I agree with the other things SS wrote. I think it's a bit too much for the existing circuit. And the start-up current on the compressor may trip the type B.

 

[sparkyspike]

Sparkyspike - 1.15 Ohms on a type B? That'd be a 32A, surely?

I thought it was 46/In which would be 46/40=1.15Ω?

 

[securespark]

Isn't the max loop fopr a B40 0.93?

 

[sparkyspike]

Damn you SS, I'm going to have to go and look it up now!

 

[ColJack]

B40 = 1.15 new regs..

 

[self builder 123]

Hi

Many thanks to everyone.

So the first thing that we will need to check is the maximum loop impedance from the B40 in the house out to the workshop.

If this is greater than 1.15 ohm does this mean that the B40 would not trip and thus be unsafe ? Is my understanding correct ?

To get around the problem of overloading the cable and thus tripping the B40 in the house could we install some form of relay system which would temporarily shut down the 2 x 3Kw fan heaters whenever the 2.2kW air compressor switched on ?

 

[NotHimAgain]

To get around the problem of overloading the cable and thus tripping the B40 in the house could we install some form of relay system which would temporarily shut down the 2 x 3Kw fan heaters whenever the 2.2kW air compressor switched on ?

The compressor may trip the mcb because its starting current might be 'seen' as a fault. This is not an overload but a response to a potential fault current.

As I said above, this is most likely to occur when you first switch on, as everything is calling for power at the same time, and there will be a certain amount of 'inrush' current into cold resistive loads (heaters etc) and inductive loads (compressor).

You might prevent mcb operation by simply switching things on in a sequence that includes a short delay between each appliance being energised.

If you have problems a relay system that shuts down the heaters whilst the compressor is charging its air vessel might do the trick - but I would leave that until after you have tried things - the problem may not occur.

 

[self builder 123]

Hi NotHimAgain

Many thanks for your reply.

If the loop impedance from the house to the workshop via the B40 is less than 1.15 ohm could we use a Type C MCB in the local consumer unit in the workshop for the compressor ?

 

[NotHimAgain]

The loop impedance would have to be less than 0.57 ohms (measured value less than 0.46 ohms) for a generic Type C mcb.

This is a limit value - a much lower impedance would be better.

You may have to consider using an RCD for fault protection instead. You can get an RCBO (mcb combined with an RCD) that is type C on its circuit breaker functions.

However, the above should not be seen as a circuit design as full details (and loads of money) are required for that

 

[self builder 123]

Hello again NotHimAgain

I should have said that the local consumer unit in the workshop will be a split load type with a 30mA RCD incorporated to protect :

- the air compressor ( via 20A MCB - dedicated circuit )

- the water heated ( via 16A MCB - dedicated circuit )

- the ring main ( via 32A MCD )


Does this alter things ?

I really appreciate your help.

 

[NotHimAgain]

Well whether it alters things or not would depend on the overall design of the installation. I will not provide a 'design' as such because to do that properly requires all the details and my professional indemnity insurance company would not be happy if I worked with incomplete information.

However, I am happy to give you some things to consider, and then you must finalise things yourself.

We are considering the distribution circuit that supplies the workshop from the house. I am assuming that you have determined the load you wish to supply and that you have applied any allowance for diversity that you think is appropriate. Diversity has been mentioned by others but it is a complex subject and something of a 'black art', especially when the loads involved are non-domestic.

To keep things short I am assuming that your overload protection provided by the 40A Type B mcb is OK. This will also be true for a 40A Type C device.

What may be at issue is shock protection rather than circuit protection. We must meet a specified disconnection time in order to comply with BS 7671. This is a distribution circuit so the time limit is 5 seconds or 1 second depending on the earthing system, but as we are using an mcb this is rather academic as disconnection would normally occur in 0.1 second or less.

These times will only be achieved if the loop impedance at the end of this circuit is within the limits I stated above. If this cannot be arranged fault protection (i.e protection against electric shock due to faults to earth) could be provided by an RCD rather than by an mcb.

There are a number of ways this could be done, each with different cost / benefits. Leaving aside the possibility of using a 'class II' circuit (don't go there - most in the UK are not ready for that yet )- all require an RCD at the house end of the circuit. This RCD must disconnect the distribution circuit in the event of an earth fault on it, or at its load end, within the specified time (5 seconds for TN systems and 1 second for TT).

In an ideal world this RCD should discriminate with any downstream device (this could be an expensive problem with your split board device) but this is only necessary to prevent danger, or to provide for 'proper functioning' of the installation.

If you are not worried about discrimination you could have a 30mA device in the house and, subject to type of earthing and availability, this might be a single module RCBO 40A Type C. If you are worried about discrimination you will need an S Type RCD (time delayed, 100mA or possibly higher if Zs permits) RCD and you might need a separate mcb and an enclosure.

 

[self builder 123]

Hi NotHimAgain

My sincere thanks. I will discuss there points with the electrician to ensure that the installation is safe.

Thank you once again.