[Question] Circiut Breakers

[Martinco]

Just a question.....................say a circuit breaker at the main DB is rated at 20 amp and a circuit breaker at the sub DB 50 meters away is also 20 amp and of the same make and type. These breakers only supply one appliance and no other apparatus. In the case of an overload, which breaker will trip first ?
Further, is there any significance in a breaker with an orange lever ?

[Justloadit]

Theoretically they will both trip on the overload.

The orange lever basically tells you that the circuit breaker is not electro magnetic, but thermal overload. These breakers are usually used in circuits on which motors are connected, so that they do not trip when the motor is started.

Some of our learned friends on the forum will give a more detailed explanation.

[AndyD]

In reality it would be a lottery which one trips first, it would depend very much on exactly what type of fault occurred and how big the fault was. If it was a short circuit at the appliance then in theory the breaker closest to the fault should trip faster because under those conditions the resistance cabling acts as a load. That doesn't mean however that both breakers wouldn't trip.

[Martinco]

Lets say the fault is simply an overload.

[murdock]

orange breaker has a slower trip curve 1 and a standard white lever is curve 2...the orange one would have a variety of uses one being to accomadate the start of lets say an aircon compressor...pool pump etc.

what i have experienced recently...is they cannot operate correctly with invertor aircons...the aircons cause them to buzz...the suppliers response was thet there is a spring around the coil which is making the noise...

my question would why would you want to install 20 amp circuit breakers in line with each other...thats like putting 2 e/l unit in line.

[AndyD]

If it was a marginal overload the white lever breaker would trip before the orange lever breaker depending where the overload is on their curve of operation. I don't have data sheets immediately available but from memory there's a point where the two tripping curves meet. If the value of your overload coincided with this value on the curves then we're back to the lottery principal again.

[Martinco]

No, just a hypothetical question to set some brains in motion.

So what I can figure out from above answers is that if you have an orange CB at the main DB and a white CB at the sub and you have a motor at the end drawing close to 20 Amp for a short amount of time then the white one will trip first ? But if the motor draws 20 amp for a long time then it is a toss up as to which breaker will fall first. Right ?

[AndyD]

I apologize in advance if this is long winded and over simplified but I thought it might be helpful for the non-electrical people. Also this info was taught to me during my apprenticeship and refers to the old way that things were done before complex electronics were thrown into the mix.

Breakers (MCB's) have two distinct tripping functions. High fault currents cause tripping by magnetism which happens very quickly, marginal overloads cause tripping by thermal action which happens much slower or delayed so it's effectively a two-function device.

As a rule of thumb;
A ‘C-curve’ breaker (white lever) has an instantaneous trip if the current through it is in excess of five times the rated current (In).
A ‘D-curve’ breaker (orange lever) has an instantaneous trip if the current through it is in excess of ten times the rated current (In).

So if you effectively have both types of breakers (C+D curve) rated at 20A protecting a circuit and a fault occurs causing over 200A to flow then both should trip instantaneously. The instantaneous tripping is performed internally in the MCB by magnetic action.

The thermal tripping function of the breakers occurs when the overload is marginal (< 5x In for C-curve and < 10x In for D-curve). This marginal overload current causes heating of a small thermal element which operates a bimetallic latch causing the trip. Playing around with the performance of the heater and the composition and construction of the bimetallic strip causes different tripping characteristics for current against a time scale. They take a breaker and plot a graph or the tripping times for different currents and the resulting line on the graph is the tripping curve of that particular breaker.

Below is a typical graph with the curve for a C-curve and a D-curve breaker. If you follow across from the point where the two curves meet it indicates the current would be approx 1.2 times the current rating of the breaker. So if you had a 20A C-curve and a 20A D-curve MCB protecting the same circuit and the current in the circuit was 24 Amps then in theory both breakers would trip at the same time.

Please note all figures and curves shown are fictional and were drawn free-hand, they should not be used to make any real life calculations. The manufacturers of any MCB should be able to supply similar graphs on request.

[Justloadit]

Hi Andy,

A very well thought out explanation, had forgotten some of the characteristics of the D-Curve CB, and even of the C-Curve

[murdock]

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.

[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