Basics on selecting a circuit breaker for FULL MOTOR PROTECTION (tech).
**This customer insists he CAN use a normal circuit breaker for FULL motor protection!!
First lets us qualify the title: I am not referring to Motor Protection Circuit Breakers for they are specifically designed for this job. We are referring to a normal traditional circuit breaker for motor protection.
By the way, I can’t believe this topic turned out as complicated as it did.
Why?
Because it is a misnomer to want to select a circuit breaker for “motor protection”. Circuit breakers are not selected for motor protection because we use overloads for that purpose…Yet nowhere does any standard/code explicitly say you cannot use it as motor protection.
The customer insists: “But the NORMAL circuit breaker has a current rating and technically it does qualify as an overload too!
I saw in your catalog – there are even adjustable MCCB – why can I not use that?”
I guess 70% of electricians already shake their heads with a twitch of the face …” does not know what he is talking about”.
But let’s just give the benefit of the doubt here, after all, there is no code in the world that specifically says one cannot use a normal circuit breaker for motor protection…but maybe they should!
In fact, the electricians “right way” is: Circuit breaker or fuses (for some) in the mains supply for short circuit protection and overload below the contactor (or standalone motor protection relay) specifically for motor overload protection. [Thanks Colin for reiterating this]
You see when it comes to motor protection there are three major considerations:
1. Short circuit protection (The PSCC of the installation)
2. Overload protection
3. Motor starting current:- When a motor starts it pulls current multiple times the motor FLA (Full Load Amps). It is this starting current that causes all this complexity.
What does the NEC say?
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430.83: (2) Circuit Breaker. A branch-circuit inverse time circuit breaker rated in amperes shall be permitted as a controller for all motors. Where this circuit breaker is also used for overload protection, it shall conform to the appropriate provisions of this article governing overload protection. Wow, that’s a surprise Article 430.52 (C)(3) says the instantaneous trip circuit breaker – simply means the magnetic part of the circuit breaker – MUST BE ADJUSTABLE…this kills any possibility for the MCB. 430.83:(3) Molded Case Switch. A molded case switch rated in amperes shall be permitted as a controller for all motors. Article 430 then allows for many, many continuous exceptions, eventually finally leaving it up to the manufacturer. 430.58 Rating of Circuit Breaker. A circuit breaker for motor branch-circuit short-circuit and ground-fault protection shall have a current rating in accordance with 430.52 and 430.110. And what does 430.52 say? A normal squirrel cage up to max 800%FLC for magnetic (instantaneous) mechanism cb and max 250%FLC for the overload (thermal) part. Really?
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and the IEC?
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Surprisingly there is no explicit allowance that says a MCB or MCCB can be used as complete protection device. It says: The SCPD (Short Circuit Protection Device [which we all know is the circuit breaker – off-course elsewhere in the world could be fuses too] and overload relay must be separate. Then the IEC just looses it with IEC 60947-4-1 : 2000 – that committee must have been on a different planet simply forgetting that even this circuit is still a subcircuit in a normal reticulation. |
but I cant resist the South African contribution…I like the South African SANS version – Short, sweet and refreshingly easy:
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SANS 6.16.5.1.3 For motor protection: c) prevent a multi-phase motor from continuing to operate under load if single phasing occurs, and) in the case of an automatically controlled motor, have to be manually reset after operation before allowing automatic restarting of the motor.
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Finally is to mention: There are manufacturers out there who wants to save you money and stay within the NORM. Allen Bradley has circuit breakers which ONLY DOES MAGNETIC TRIPPING- i.e. short circuit protection only. So you don’t fork out for a thermal feature if you don’t need it. Just thought I should mention it here. |
At this point let me already say:
Motor overloads are specifically designed for all the different sizes of motors so much so that you can adjust it to the spot for the FLA.
On the other hand MCCB has a few notches in percentages typically 0.4, 0.63, 0.8 and 1 where to adjust the tripping current. It would be most unlikely it will correspond as close as one would hope for the motor FLA.
Its design allows for a much broader setup. Additionally – I don’t know one manufacturer with an adjustable MCB.
There is only but a slight chance that one can stay within short circuit, starting current and overload requirements with a NORMAL MCB or MCCB.
But now we want to force to use a NORMAL circuit breaker:
Why?
Because the codes do allow for one to use it, or does not say it cannot be used.
So lets take every consideration and see how to qualify in terms of the different codes.
kA rating of the circuit breaker (instantaneous or magnetic part) selection.
Do not make the mistake of thinking the starting current and the kA rating (PSCC – Prospective Short Circuit Current) is the same thing).
The PSCC rating is pre-determined by the source capacity to ensure the protective equipment can handle the source energy/capacity without any damage under short circuit conditions.
But having said that, the above kA (PSCC) tripping mechanism in the circuit breaker happens to be exactly the same mechanism (magnetic trip) that will determine if the motor will start or not.
Technically this means we are trying to qualify the poor circuit breaker for two different functions at the same time. Two bosses with very different agendas.
So whatever is done in the selection don’t make the mistake of under-sizing the kA rating on the premise of the Prospective Short Circuit current of the circuit at that point. We can go higher but not lower.
But additionally:
NEC starting current /kA rating
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Under NEC table 430.52 it has to comply with the instantaneous maximum trip factor of 800%FLC (normal squirrel cage application)…though there are exceptions to increase this value. 430.52 (C)(3) says: for an instantaneous trip circuit breaker only an adjustable can be used. But this type of circuit breaker is extremely rare in my world. |
IEC:
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If the CB kA rating is higher then the starter kA rating then it will qualify as a protection device but if it is smaller then you have to do a short circuit test. (I hope they meant: The PSCC has to be determined at that point)
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Overload selection:
NEC says:
| Max 115%FLC and under worse circumstances up to 140%FLC it must trip. Strangely there is no mention of time, although the NEMA spec is mentioned and there is the additional requirement where the overload must trip within a designated time period according to the overload classes. Very strange. |
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IEC says: Between 8.2.1.5.1, Tables 2 and 3: At values from 1 and 105% FLC an overload must trip in over 2hrs and at values of 120 to 130%FLC but at 150%FLC it must trip for Class 10A,10, 20 and 30 – tripping shall occur in less than 2, 4, 8 or 12 min. |
And then comes the one I always get: If we can’t use a NORMAL MCB or MCCB then what is this D-curve circuit breaker for?
The nice thing about the D-curve circuit breaker is that it takes longer to trip so one can have a much lower-rated current circuit breaker and still allow the high motor starting currents.
Installations with C- curves will have to overrate the circuit breaker by multiple times motor FLA (like 3 times) to prevent tripping when the motor starts. This means bigger wires etc.
By the way: – MCCB (Molded case circuit breakers) are by design are “D curves” and even better is the fact there are adjustable models.
With a bit of luck one can then adjust the magnetic trip (Short circuit protection) and the thermal (Overload protection) to the point where it qualifies as a motor protection device. But you will have to know what you are doing. Interpret the tripping curves so you stay within the tripping classes specified by the codes and even more importantly the motor manufacturers’ recommendations.
PS: What does it mean “You have to know what you are doing?” – Am I being funny or what.
I am not being funny at all…let me explain.
Take the tripping curve of the circuit breaker (Magnetic and thermal).
Take the starting curve of the motor.
Superimpose the two curves on say graph paper – or software if you have.
The two curves of which the motor starting which must always stay on the left of the CB tripping curve, should never intersect.
Focus on the overload part (thermal overload) and ensure the setting of the potential circuit breaker is running in accordance of:
- What the motor manufacturer requires.
- What the standard requires.
- Ensure the wiring and component requirements on the site is covered.
Finally, ensure the minimum kA rating at that point.
There we are done…now we must just find the circuit breaker.
Does this qualify as “You must now what you are doing?”
Look, at the end of the day, the purpose of this article is not just so much as to prove or disprove an MCB/ MCCB as a motor protector but to bring across what the basic requirements are regarding motor protection.
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