Circuit breakers

The mcb rating chosen is fitted for the size of cable used. 20A typically for 2.5mm radial in the UK.

Ah! Thanks. So with my 10mm cable I should probably consider a change. Will the 20 amp be holding things back do you think?

I doubt it will change the sound but if it were me I'd still have a more appropriate mcb fitted. This will depend on how the cable is fitted as running a cable through conduit or around insulation for example will de rate the cable. Best have a chat with the sparky. 

Some people here have mentioned 50A. I went for 32A type C, rather than the more common type B, and 10mm T+E cable. This was as recommended to me by Naim support, and my electrician was happy to instal it. 

For clarity (and safety) sake, I assume you are UK based?

Richard Dane posted:

For clarity (and safety) sake, I assume you are UK based?

Yes, I'm in the UK. I think the OP is, too, but with electrical safety, of course one should never assume anything.

ChrisSU posted:

Some people here have mentioned 50A. I went for 32A type C, rather than the more common type B, and 10mm T+E cable. This was as recommended to me by Naim support, and my electrician was happy to instal it. 

I think the advice given must be based on locale, circumstance and wiring gauge. The current rating will be based on the gauge of the wire, and the letter is the inductiveness of the total load. I must admit if I had many large Naim transformer devices on a ring or a radial I would be tempted for a high inductive load type D  if you can - this gives 10 to 20 times the quiescent current - as opposed to type C which is medium inductive load of 5 to 10 times the quiescent current. This way you will minimise annoying trips on power glitches etc

Since installing this , my 200 switches on first time, every time. I suspect this is down to the type C MCB, as the old circuit was a 16A type B, and this often tripped when I tried to turn on the power amp.

Indeed a single, two, three or four large torodials should be fine - above that a Class D might be beneficial

fitted over rated mcb for the cable is totally wrong and dangerous. ensure that you use a  breaker from an know source some known to be not safe.  local suppliers in your area will give you the correct advice

indeed the current rating of the MCB must be aligned to the cable size. The Class rating is based on the reactive element of the load and therefore is based on the transition surge when a reactive load is suddenly enabled. Higher reactive loads need to have a higher ratio of instantaneous peak at switch on to quiescent current on the MCB, as reactive loads will always resist sudden change. Reactive loads due to change are many times higher albeit very brief load on the circuit compared to the quiescent state. That is why when you can often see when large motors  or transformers are suddenly switch on you can see a momentary dim or flicker on nearby circuits. 

Therefore you also don't want to compensate with a higher value MCB current rating to offset the reactive surge as that would be unsafe - hence the class ratings

S

Hi,

I'm new to the forum but may I offer my two pennies worth on this topic as MCBs were part of my one of my previous jobs.

Certainly the cable size is a significant factor in determining the nominal MCB rating that should be used. Overall though the MCB rating should be selected to protect the cable based on current carrying capacity of the cable as installed. Buried, surface mount, conduit, insulation, expected room temperature... all have an effect.

B, C or D curve is, as already explained, chosen to satisfy the inrush of current that all non-resistive circuits exhibit on switch on (some resistive circuits also see inrush currents), but also to satisfy the value of earth impedance within your building so the circuit breaker trips any  over current condition within specified times.

I have not looked into the reactance of MCBs but would guess that D-curve breakers would have less reactance than C-curve so may have less effect on transients. Manufacturers of MCBs do produce tables of 'Watts lost' in an MCB and this would give an idea of the resistance of the MCB as a whole. Larger current rated MCBs are likely to offer a lower watt loss. Will it make a difference? Can't say.  If selecting protective devices is an issue would cartridge fuses be preferable?  I have no idea if fuse or circuit breaker would offer a noticeable difference in overall sound but my general thought is 'simpler the better'. Not much simpler than a fuse.

simon.