MCB Energy Let Through Values

steve100 · Mar 6, 2013

I know that MCB's are grade with an energy let through rating of 1, 2 or 3 and 3 being the highest. I also know that it relates to the sizing of the cpc. However that is about where my understanding ends, I have read a few threads and documents, but still having trouble getting my head around it. Could someone please explain this principle in simple layman terms so I can understand?

Also in the current OSG it states that for MCB's of <=16A at a short circuit capacity of 6KA the min cpc should be 2.5mm2, but a lighting circuits cpc is 1.5??

Thanks

steve100 · Mar 12, 2013

Just bumping to top for answer

johnboy6083 · Mar 12, 2013

The let through energy is called I2t (Current x current x time)

The equation for sizing a CPC uses the square root of this value divided by a constant (called K and given in section 5 of BS7671)

for some devices the i2t is sometimes called A2s (amps x amps x seconds)

steve100 · Mar 12, 2013

Also in the current OSG it states that for MCB's of <=16A at a short circuit capacity of 6KA the min cpc should be 2.5mm2, but a lighting circuits cpc is 1.5??

Thanks

So when would you a rating of 1, 2 or 3?

In relation to the above point, why does the OSG state that the minimum cpc of 2.5mm2 should be used for circuits supplied by a <=16A at a breaking capacity of 6KA, but we use a 1.5mm2 cpc for a lighting circuit which contradicts the above?

Thx

johnboy6083 · Mar 12, 2013

I haven't got my OSG to hand to see the context in which the above is mentioned. The CPC is calculated using the adiabatic equation, or by selection using table 54.7

Paul.M · Mar 12, 2013

Hello Steve. May I/We just make a point or two?

1. There is no such thing as lighting cable. It is a cable that can carry a certain amount of current for the load intended via design for the load. In you case "lighting". eg:- You can not feed a flood light for a football pitch with 1.5mm. Please get the term of :- "lighting cable" out of your head.

2. 1.5mm T&E comes with a 1.0mm CPC.

We design the circuits to suit the load and the tables in the regs. Never call any csa of cable "lighting, sockets or mains cable", thats down to us to choose what cable we use. I'll post up some pictures of what I'm working atm just to make you all think a bit. First time I've seen this form of lighting and I'm looking forward to installing it, it will be interesting. (LCM Lighting Control Moduals for the people that have fitted them). Good fun and let me have a go at it. Its only a box + a load of wires. What could possibly go wrong?

That is the thing about us Electricians, we can work things out regardless of the situation that we are presented with. Through our training/knowledge/experience we can resolve most problems. THAT IS A TRUE ELECTRICIAN. This is what we all try to achieve.