Maximum settings for instantaneous breakers in combination starters

[mull982]

The NEC says that for an instantaneous CB used as part of a listed combination starter, the adjustable setting on the CB may be a maximum of 8x the motors FLC, or if proven by engineering review up to 17x the motor FLC. My question is why are these maximums chosen, and what advantage is there for setting the breaker as low as possible? Obviously it would allow better protection, but what are you risking or giving up by dialing the breaker to higher values?

One example I have been discussing recently involved a motor with a .75A FLC. Someone wanted to use a 7A breaker and I explained to them that this was too big and was a code violation because even the lowest setting on the 7A breaker is (3x rating) 21A which is much greater than the absolute max of 17x FLC allowed for the motor. It is even much greater than the 8x FLC you should start with.

I am saying that this motor needs a 1A breaker (if made in that size) this way the breaker is adjustable from 3A to 10A and is within the allowable range. The more I think about it though I'm thinking that maybe a 3A breaker should be used to allow higher adjustments if necessary however even the lowest setting of 9A would be greater than the 8x your supposed to start with. I'm not sure of the typical increments of breaker sizes for this type of MCP breaker so I wanted to hear other opinions.

 

[Jraef]

What you risk by setting it too low is nuisance tripping. Any perceived "better protection" is irrelevant in that this is short circuit protection, not overload protection, so "better" would mean what? It trips a few milliseconds earlier on a short circuit? The difference in damage caused is not going to be noticed; if you have a short circuit or grounded fault, the damage has already happened. All you are trying to do now is prevent a fire.

As to your .75A motor, read the NEC more closely; fractional HP motors fall under a slightly different set of rules. You could have had a 15A breaker in there.

 

[templdl]

The way I look at it is that the MCP must not nuisance trip on start up. The purpose on the MCP is to take the motor off line should there be a winding failure. When one refers to better protection a winding failure is a winding failure. If the MCP is set just above the nuisance trip point one could consider that should the MCP trip that there is a winding failure. If set higher the MCP would need a greater fault current to trip. So one may conclude that the lower the MCP is set without nuisance tripping it would provide better protection. But from what? The windings still failed but it may result in less motor winding damage. Also, I believe the NEC refers to the MCP as ground fault protection because the winding when they do fail, fail to ground resulting in a ground fault.

 

[Jraef]

... The windings still failed but it may result in less motor winding damage.

What I meant was that a few milliseconds one way or the other is not going to change the outcome significantly. "Less" winding damage has the same general effect as "more" winding damage, i.e. the magic smoke comes out and the motor must be rewound.

... Also, I believe the NEC refers to the MCP as ground fault protection because the winding when they do fail, fail to ground resulting in a ground fault.

It does provide grounded fault protection (not to be confused with other forms of "Ground Fault" protection schemes) but you can have a turn-to-turn failure as well.

 

[mull982]

What you risk by setting it too low is nuisance tripping. Any perceived "better protection" is irrelevant in that this is short circuit protection, not overload protection, so "better" would mean what? It trips a few milliseconds earlier on a short circuit? The difference in damage caused is not going to be noticed; if you have a short circuit or grounded fault, the damage has already happened. All you are trying to do now is prevent a fire.

If you are saying that the purpose of this breaker is just to isolate faults and prevent fires once the damage is already done, the why does the NEC put restrictions on the maximum settings? If we are saying that setting them as low as possible provides no benefit then why care how high they are set?

As to your .75A motor, read the NEC more closely; fractional HP motors fall under a slightly different set of rules. You could have had a 15A breaker in there.

I see this exception that you are referring to. It looks like we can have a 15A breaker that can be increased to the value marked on the controller. I dont follow what is meant by "the value marked on the controller"? Does this mean that we can set it to the maximum?

Even the minimum setting of 3x on this breaker would be 45A which would be much mure than 8x or 17x the rating of the motor.

 

[cschmid]

if you are actually worried about the fractional hp motor why are you not considering fuses...or is this just about the wording on a design spec..

 

[Jraef]

If you are saying that the purpose of this breaker is just to isolate faults and prevent fires once the damage is already done, the why does the NEC put restrictions on the maximum settings? If we are saying that setting them as low as possible provides no benefit then why care how high they are set?

The value has to be set at something, but the level isn't as important as what you you might think. If you look, they essentially say 250%, or 700%, or 1700%, whatever works. If they had NO value set in writing, someone would put a 400A breaker on a 1/2 HP motor, and THAT might end up being a problem. But in a breaker, the opening time alone is too long to prevent the motor windings from being damaged, that's why I said it's role as a SCPD mainly to prevent a fire at that point.

I see this exception that you are referring to. It looks like we can have a 15A breaker that can be increased to the value marked on the controller. I dont follow what is meant by "the value marked on the controller"? Does this mean that we can set it to the maximum?

Even the minimum setting of 3x on this breaker would be 45A which would be much mure than 8x or 17x the rating of the motor.

I don't have my NEC here with me so I'll have to read it later to see what you are seeing.

 

[kingpb]

The MCP, which you are referring to as a CB, only has magnetic trip function capability (instantaneous). This is why it is not useful as overload protection (thermal). The magnetic function is viewed as an instantaneous trip setting, and it's setting needs to be low enough to initiate a trip when it senses a level of current that it was set for.

You need to make sure that it is set above the highest value of current that the motor will draw on starting. Checking the motor starting curve is essential for this. As an example, say the motor will draw a maximum 600A on start, and your short circuit analysis indicates the fault current level to be 5,000A. Then setting MCP to trip anywhere from say 1.25x the 600A (750A) up to say 5X the 600A (3000A) will result in the same thing; protection of the circuit from a fault condition. What you want to avoid is setting the trip to low; within the error band which may be +/- 5% of 600A (570-630A) because this could mean you could still trip on "600A" (nuisance), or too high; above 5000A, because the actual fault may not produce enough current to make the MCP initiate a trip.

The OL setting (thermal setting) will protect the motor and cable from an overcurrent situation. The combination of the two devices, replaces the CB which has both magnetic (instantaneous) and thermal (overcurrent) even though the motor OL protection also provides the thermal protection. The OL are more closely set to the motor characteristics.

Hope this makes some sense.