Circiut Breakers

[Martinco]

Thanks Andy, Makes sense to me now and very well put.

[AndyD]

just be careful because if you start looking on an orange CBI circuit breakers chances are you not gona find "D" curve anywhere...they are refered to as curve 2 "C2" (motor starter type breakers or eqipment with inrush currents)

circuit breakers like M&G are refered to as "D" curve (motor starting breakers or equipment with inrush currents) and all the ones i have installed have always been black.

You're right, not all d-curve breakers have an orange lever, unfortunately there are variations between manufacturers but the curve of the breaker is invariably stated on the front of it and all major manufacturers have detailed curve information on their websites, some state the curves on the packaging.

One interesting thing I have noticed over the years is that not all breakers perform well or consistently in high or low ambient temperatures. Information about the effects of ambient temps on the breakers curve of operation are a lot more difficult to find and are often not available at all. This can present problems in a country where the ambient temp might range between +50 and -20 Celsius.

[Upstairs]

I had a contractor install a 40Amp C-curve breaker to compensate for start-up current on my 11Kw compressor. Start -up was star delta. Cable size was 2.5mm. Needless to say he will not work for me again.

[AndyD]

Did your compressor fail?

The thing that determines the size (Amp rating) of a circuit breaker is the conductor size and the length of the cable it is supplying. The machinery that is connected would not make any difference to the Amp rating of the MCB, the MCB's only task is to prevent the actual supply circuit wiring from being overloaded. The star-delta starter has a thermal overload built in that is designed to specifically protect the motor (compressor in your case). Even with a 40A mcb the thermal overload should prevent damage to the compressor being caused by high run currents if it's specified and set correctly.

Supplying a 2.5 mm cable directly with a 40Amp MCB is not legal, the breaker is way overrated (or the cable is way underrated, depending which way you look at it). If the 2.5mm cable was installed between the star-delta starter and the compressor and if the overload in the starter was set at 25 amps or less then the cable size might be within the regs. An 11Kw 3-phase motor would have a nominal full load current of around 22Amps at 380-400 volts so a 4mm cable may have been a better choice but also may not have been required according to current carrying capacity tables in the regulations.

From the description you gave of the installation you may (or possibly may not) be condemning your contractor unfairly, it's impossible to say without a complete and accurate schematic of the full installation.

[murdock]

supplying and installing a 40 amp circuit breaker would be a indication of the limited knowledge of the contractor....but other than that i dont see a problem especially if the compressor is like the one i connected on wednesday...where the compressor reaches pressure in 1 and a half minutes...it was a 7.5 kw 3 phase unit with 3 pistons...and runs at a much slower rpm than most i have seen...i could have got away with a 1.5 mm wire and a 15amp (curve 1 or D curve) breaker...the compressor was only running at 13 amps under load...

however baecause i wired this factory many years ago there is a 4 mmx 4 core + e wire to most of the isolators in the factory...the reason i do this is because of whatalways happens in factories...machines move...get sold and newer ones are installed etc and you just never know what will be connected ...i never have to worry about rewiring circuits...the cost is only slightly more but in the long run we save the factory thousands...we dont have volt drops issues...its a win win situation....most machines we connect are normally with the 30 amps allocated...the only time we have to upgrade to bigger cables is when big CNC machines etc are purchased...but then you move up to the 80 amp per phase range which would start really costing to make provision