Magnetic only breaker

[mbrooke]

What does the time current curve look like for a magnetic only breaker? Will it trip at 134% of its rated current? And how fast? I see nothing in the code which requires a breaker have thermal protection.

 

[adamscb]

I don't know about the specific breaker in the picture, but with magnetic-only HMCP breakers it will trip instantaneously if, and only if, the current reaches a certain point. This setpoint is adjustable on the front of the breaker, and the setpoint is usually pretty high.

It's designed for short circuits or inadvertent grounding of equipment. On the other hand, thermal protection is designed to protect against overloading of the circuit, and not short circuits or grounds. ​

And there is no code requirement that says the circuit breaker has to have both...but you have to have both in your circuit, whether it comes from the circuit breaker or elsewhere.

I work in the industrial three-phase world, and in most of our MCC starter enclosures, the breakers are magnetic-only and protect against the motor shorting out or going to ground, or extreme overloads. Meanwhile the "heaters" protect against less severe overloads, and thus provide thermal protection according to FLA of the motor.

 

[mbrooke]

I don't know about the specific breaker in the picture, but with magnetic-only HMCP breakers it will trip instantaneously if, and only if, the current reaches a certain point. This setpoint is adjustable on the front of the breaker, and the setpoint is usually pretty high.

It's designed for short circuits or inadvertent grounding of equipment. On the other hand, thermal protection is designed to protect against overloading of the circuit, and not short circuits or grounds. ​

And there is no code requirement that says the circuit breaker has to have both...but you have to have both in your circuit, whether it comes from the circuit breaker or elsewhere.

I work in the industrial three-phase world, and in most of our MCC starter enclosures, the breakers are magnetic-only and protect against the motor shorting out or going to ground, or extreme overloads. Meanwhile the "heaters" protect against less severe overloads, and thus provide thermal protection according to FLA of the motor.

But how do they accomplish a 40amp rating? Either the breaker instantly trips at 70 amps (impractical) or only at 400amps (dangerous).

 

[ptonsparky]

The very few I’ve worked with tripped at the ‘40’ unless adjusted by the dial. Those were built in the 60- 70s. IDK if the time or magnitude was adjusted.

 

[romex jockey]

.... found in the '17 definitions>>>>

Electronically Actuated Fuse. An overcurrent protective device
that generally consists of a control module that provides
current sensing, electronically derived time–current character
istics, energy to initiate tripping, and an interrupting module
that interrupts current when an overcurrent occurs. Electroni
cally actuated fuses may or may not operate in a current
limiting fashion, depending on the type of control selected

not sure if it fits here....:?~RJ~

 

[Jraef]

I see nothing in the code which requires a breaker have thermal protection.

It’s there, it just doesn’t use those words and partially because the Code is not specific to technology, just function. It’s referred to as “inverse time-current” over current protection, meaning that the higher the current, the shorter the time. That effectively describes the “thermal” portion of what’s sold as a “Thermal-Magnetic” circuit breaker. But you can also achieve this electronically or even hydraulically, so the Code just says what it needs to DO, not how to do it.

The Magnetic portion is strictly for short circuit protection ONLY, so with a Magnetic-Only (Mag-Only) breaker, you do NOT have the inverse time-Current tripping capability. This is why Mag-Only breakers are ONLY allowed to be used as “Motor Circuit Protectors” and are ONLY allowed to be used as part of a factory built, tested and listed combination starter assembly. You CANNOT use them on your own for anything. Period. The only reason they are sold to the general public is as DIRECT ONE-FOR-ONE REPLACEMENTS. In the NEC the more generic functional term they use for them is “Instantaneous Trip” circuit breakers.

So will it trip on a gradual increase in current to 134%? Absolutely NOT. There is nothing in an MCP that will sense a gradual increase, it’s ONLY function is to immediately trip at the short circuit current level you set it for, which can be as high as 1700% of your motor FLA. This is why you can ONLY use them as part of a factory built combination starter, because that starter will have an Over Load Relay as part of it, so that OLR is going to provide the inverse time-current trip functionality that is required in the circuit. The difference is that the setting of the OLR will be closer to the motor FLA, not the fixed value in a CB used just to protect conductors, but that will ALSO protect the conductors anyway, hence not needing it to be redundant in the breaker.

When an MCP says it is “rated” at a value, such as 40A, that is just the continuous current rating of the breaker, mostly having to do with the terminals and the mag trip elements inside. The trip adjustment itself will range typically from 300-1000% of that value. So a 40A MCP will be adjustable from 120 to 400 amps

 

[dkidd]

What does the time current curve look like for a magnetic only breaker? Will it trip at 134% of its rated current? And how fast? I see nothing in the code which requires a breaker have thermal protection.

Here are some curves. They trip instantly at the set current.

 

[steve66]

Here are some curves. They trip instantly at the set current.

Interesting and educational.

From the curves, it looks like the general idea is that if the breaker is set at 40 amps, then it would never trip at anything below 40 amps, but it would trip as fast as possible for anything over 40 amps.

That's not what I would have expected from a MCP, or a magnetic only breaker, and it doesn't seem like that would work very well for allowing motor starting currents. I assume one would have to select a breaker with a setting above the motor starting current.

 

[dkidd]

Here is an MCP and overloads and motor starting current.

 

[mbrooke]

It’s there, it just doesn’t use those words and partially because the Code is not specific to technology, just function. It’s referred to as “inverse time-current” over current protection, meaning that the higher the current, the shorter the time. That effectively describes the “thermal” portion of what’s sold as a “Thermal-Magnetic” circuit breaker. But you can also achieve this electronically or even hydraulically, so the Code just says what it needs to DO, not how to do it.

The Magnetic portion is strictly for short circuit protection ONLY, so with a Magnetic-Only (Mag-Only) breaker, you do NOT have the inverse time-Current tripping capability. This is why Mag-Only breakers are ONLY allowed to be used as “Motor Circuit Protectors” and are ONLY allowed to be used as part of a factory built, tested and listed combination starter assembly. You CANNOT use them on your own for anything. Period. The only reason they are sold to the general public is as DIRECT ONE-FOR-ONE REPLACEMENTS. In the NEC the more generic functional term they use for them is “Instantaneous Trip” circuit breakers.

So will it trip on a gradual increase in current to 134%? Absolutely NOT. There is nothing in an MCP that will sense a gradual increase, it’s ONLY function is to immediately trip at the short circuit current level you set it for, which can be as high as 1700% of your motor FLA. This is why you can ONLY use them as part of a factory built combination starter, because that starter will have an Over Load Relay as part of it, so that OLR is going to provide the inverse time-current trip functionality that is required in the circuit. The difference is that the setting of the OLR will be closer to the motor FLA, not the fixed value in a CB used just to protect conductors, but that will ALSO protect the conductors anyway, hence not needing it to be redundant in the breaker.

When an MCP says it is “rated” at a value, such as 40A, that is just the continuous current rating of the breaker, mostly having to do with the terminals and the mag trip elements inside. The trip adjustment itself will range typically from 300-1000% of that value. So a 40A MCP will be adjustable from 120 to 400 amps

I would fully agree- but I did once break open a 1 inch crouse hinds breaker that only had a solenoid coil and no bimetal that I could see. Though to be fair it may have been hydraulic...


Anyone know where I can get the time current curves to those old 1 inch Crouse Hind breakers?

 

[mbrooke]

Here are some curves. They trip instantly at the set current.

View attachment 23312

Neat, thanks

 

[paulengr]

There are kind of three schemes when it comes to starters. Let’s start with type 2 protection. Type 2 was originally an IEC thing but now shows up in US standards. In this case the motor, wiring, and starter are protected from damage under all circumstances except very light contact welding. The scheme requires current limiting fuses and an overload relay in the general case. NEMA has lists of allowable fuses. Other schemes that limit current can be used to achieve the same thing but it’s very situation specific so the Codes don’t cover it.

If we allow the starter to be destroyed during a big short circuit then the next best scheme uses an MCP and an overload relay. Or we can use current limiting fuses just not type 2 style protection. These schemes protect against damage from shorts, overloads, and single phasing and trip in all cases but the starter might get welded shut or destroyed.

The next best scheme uses noncurrent limiting fuses or a standard breaker. There is no good scheme here. Set the fuses or breaker high enough to avoid nuisance trips and full protection is lost. Set it low enough to give good protection and nuisance trips can happen under heavy starting conditions. This is with traditional schemes. A microprocessor overload relay can cover some cases to where it can cover some cases such as single phasing that the high breaker/fuse setting misses.


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[Jraef]

Interesting and educational.

From the curves, it looks like the general idea is that if the breaker is set at 40 amps, then it would never trip at anything below 40 amps, but it would trip as fast as possible for anything over 40 amps.

That's not what I would have expected from a MCP, or a magnetic only breaker, and it doesn't seem like that would work very well for allowing motor starting currents. I assume one would have to select a breaker with a setting above the motor starting current.

No, and I'm not sure how you are getting that. There is nothing in an MCP that will trip at any value lower than the lowest setting of the magnetic trips, usually 300% to 500% of the breaker rating (on Eaton HMCPs, the lowest setting is 500%, on some others it is 300%). So looking at an Eaton HMCP, they don't make a 40A, but the 50A version only allows settings of 250A at the lowest, 500A at the highest. So at anything under 250A, it will NEVER trip.

 

[mbrooke]

No, and I'm not sure how you are getting that. There is nothing in an MCP that will trip at any value lower than the lowest setting of the magnetic trips, usually 300% to 500% of the breaker rating (on Eaton HMCPs, the lowest setting is 500%, on some others it is 300%). So looking at an Eaton HMCP, they don't make a 40A, but the 50A version only allows settings of 250A at the lowest, 500A at the highest. So at anything under 250A, it will NEVER trip.

Would you say these start to trip at 300-500%?


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I can get entire lines of breakers 6-125 amps with only a coil.

 

[Jraef]

I wouldn’t trust them to trip at all...

No mention if they are thermal mag or mag only. But no UL listing and only 3kA interrupt capacity? There is no place to use those unless you have another current limiting device ahead of them, so what’s the point?

The Chinese will sell you anything you want to pay for. It’s up to you to know what they are and whether they are good. In this case, you have absolutely no way to know. No thank you.

 

[mbrooke]

I wouldn’t trust them to trip at all...

No mention if they are thermal mag or mag only. But no UL listing and only 3kA interrupt capacity? There is no place to use those unless you have another current limiting device ahead of them, so what’s the point?

The Chinese will sell you anything you want to pay for. It’s up to you to know what they are and whether they are good. In this case, you have absolutely no way to know. No thank you.

Mag only.

These wind up in installations over seas. But you are correct, these might not trip even at 3000amps :lol:

I ask because I've taken apart a 1 inch Crouse Hinds resi breakers that only had a magnetic trip coil along with a data sheet confirming that. Had I known then what I know now I would have kept it. But somehow no one wants to take me seriously.

 

[romex jockey]

So how would '3rd party testing' apply here? :?~RJ~

 

[SceneryDriver]

So how would '3rd party testing' apply here? :?~RJ~

Your client is the 3rd party. If their building burns down, the device failed the test. :blink:


SceneryDriver

 

[petersonra]

the code does not require breakers to have a time delay at all. it does require circuits to have both short circuit and overload protection. how that is done is a technology issue.

 

[paulengr]

So how would '3rd party testing' apply here? :?~RJ~

Mag only "breakers" are not circuit breakers in Code they are motor circuit protectors, MCPs. The most popular brand (Eaton) calls them thst, aka HMCP series, but technically they are not circuit breakers. The adjustment on them is in Amps and sets the magnetic trip. The rating such as 100 A isn't even printed on the handle like an actual circuit breaker.

Third party testing is per the UL standard but just as with say SIS or AWM wire, it's basically a pass through where the equipment builder ultimately submits the assembly such as a combination starter for third party approval. There is a lot of equipment out there that is UL or some other third party Listed but not suitable for general use so can only be used as part of a Listed assembly.


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