svaurez
Member
- Location
- California
Is there any reason to install a fused disconnect along with heaters or is this just overkill?
motor starter
- Thread starter svaurez
- Start date
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bphgravity
Senior Member
- Location
- Florida
Re: motor starter
The purpose of a fuse or a circuit breaker is for OVERCURRENT protection. The purpose of a heater is for OVERLOADING protection. The difference is the fuse or breaker is there to protect the wiring in the circuit whereas the overloads or heaters are there to protect the motor, usually from overheating. Both are required in most cases per Article 430 of the NEC. 430.32 for overload selection, 430.52 for the bracn-circuit OCD.
The purpose of a fuse or a circuit breaker is for OVERCURRENT protection. The purpose of a heater is for OVERLOADING protection. The difference is the fuse or breaker is there to protect the wiring in the circuit whereas the overloads or heaters are there to protect the motor, usually from overheating. Both are required in most cases per Article 430 of the NEC. 430.32 for overload selection, 430.52 for the bracn-circuit OCD.
svaurez
Member
- Location
- California
Re: motor starter
I was given a fused disconnect that contains heaters.The branch circuit is protected by a breaker.Are these fuses necessary since the breaker is protecting the branch circuit.
I was given a fused disconnect that contains heaters.The branch circuit is protected by a breaker.Are these fuses necessary since the breaker is protecting the branch circuit.
- Location
- Massachusetts
Re: motor starter
svaurez, you should read article 430 Motors.
The circuit breaker at the panel you are coming from is not overload protection it is short circuit protection, the motor overloads in combination with the fuses protect the motor and branch circuit conductors against overload/overcurrent.
In some case's you might use a 60 amp breaker with #12 awg. on a motor circuit and still be code compliant.
This is much different then what we usually do, HVAC equipment is this way also article 440
[ March 30, 2003, 08:59 PM: Message edited by: iwire ]
svaurez, you should read article 430 Motors.
The circuit breaker at the panel you are coming from is not overload protection it is short circuit protection, the motor overloads in combination with the fuses protect the motor and branch circuit conductors against overload/overcurrent.
In some case's you might use a 60 amp breaker with #12 awg. on a motor circuit and still be code compliant.
This is much different then what we usually do, HVAC equipment is this way also article 440
[ March 30, 2003, 08:59 PM: Message edited by: iwire ]
Re: motor starter
This is and interesting discussion. This is the way I view the applicaton and would appreciate your comments.
It has been my understanding that heaters are sized to protect the motor from overload as well as a locked rotor condition, the overcurrent that results from either event will cause the OLR to open the contactor. If fuses and/or breakers do protect a motor form being damaged from a locked rotor condition then one must conclude that the heaters and overload relay do not detect and open the contactor in the even of a locked rotor condition.
Personally, I'm not a fuse man and also dislike the use of thermal magnetic breakers with combination starters even though the NEC allows it. I do like the adjustable mag only type motor circuit protector (MCP), which can be set just high enough not to trip on motor startup inrush but sensitive enough to trip and open instantaneously when a current is detected just above the inrush current value, the settings of which are described in art430.52(3).
It is also my understanding that both fuses and breakers are not intended to "protect" the motor unless the motor manufactured specifically says they will. What fuses and circuit breakers will do, if selected and/or set properly, is to limit the damage to the motor should there be a motor failure and to protect the starter and motor circuit, again, in the event of a motor failure. Winding to frame or grounds often results in an arcing fault which is not sensed be the heaters. Thus, such motor circuit protection is often referred to as short circuit and ground fault protection as referred to in 2002 NEC art 430.52(3)Instantaneous trip breakers. If a fuse of a breaker has blown or tripped the motor is most likely in the process of failing or has failed. Motor failures often are the result of windings that go to ground which then may escalate to line to line. Years ago such failures often resulted in fires.
This is and interesting discussion. This is the way I view the applicaton and would appreciate your comments.
It has been my understanding that heaters are sized to protect the motor from overload as well as a locked rotor condition, the overcurrent that results from either event will cause the OLR to open the contactor. If fuses and/or breakers do protect a motor form being damaged from a locked rotor condition then one must conclude that the heaters and overload relay do not detect and open the contactor in the even of a locked rotor condition.
Personally, I'm not a fuse man and also dislike the use of thermal magnetic breakers with combination starters even though the NEC allows it. I do like the adjustable mag only type motor circuit protector (MCP), which can be set just high enough not to trip on motor startup inrush but sensitive enough to trip and open instantaneously when a current is detected just above the inrush current value, the settings of which are described in art430.52(3).
It is also my understanding that both fuses and breakers are not intended to "protect" the motor unless the motor manufactured specifically says they will. What fuses and circuit breakers will do, if selected and/or set properly, is to limit the damage to the motor should there be a motor failure and to protect the starter and motor circuit, again, in the event of a motor failure. Winding to frame or grounds often results in an arcing fault which is not sensed be the heaters. Thus, such motor circuit protection is often referred to as short circuit and ground fault protection as referred to in 2002 NEC art 430.52(3)Instantaneous trip breakers. If a fuse of a breaker has blown or tripped the motor is most likely in the process of failing or has failed. Motor failures often are the result of windings that go to ground which then may escalate to line to line. Years ago such failures often resulted in fires.
- Location
- Massachusetts
Re: motor starter
My understanding of this, which is not on Bennies level, is that the fuses react much faster to locked rotor or loss of a phase.
By their design the heaters are slow to react to these conditions, and the breaker at the origin of the circuit may be sized for high start up currents preventing it from tripping fast enough for locked rotor or loss of phase protection.
I always believed that the motor overloads where just that, if the motor is overloaded driving a mechanical load higher then it's rating the heaters will after a time trip out to protect the motor from overheating.
Any thoughts, am I on the right track?
My understanding of this, which is not on Bennies level, is that the fuses react much faster to locked rotor or loss of a phase.
By their design the heaters are slow to react to these conditions, and the breaker at the origin of the circuit may be sized for high start up currents preventing it from tripping fast enough for locked rotor or loss of phase protection.
I always believed that the motor overloads where just that, if the motor is overloaded driving a mechanical load higher then it's rating the heaters will after a time trip out to protect the motor from overheating.
Any thoughts, am I on the right track?
barrydmi
Member
- Location
- Washington
Re: motor starter
Simply put, the overloads trip when the heaters get hot enough, due to the motor being overloaded. The fuses are there to protect the motor or the circuit from short circuit and or ground fault.
Simply put, the overloads trip when the heaters get hot enough, due to the motor being overloaded. The fuses are there to protect the motor or the circuit from short circuit and or ground fault.
Re: motor starter
Bob(iwire) You do fine on all levels.
The fuses are the most important protection for any motor that may lock up from loading problems.
A motor winding will smoke a lot before the branch circuit breaker wakes up.
The overload heaters remain cold even after the fire dept arrives.
A motor circuit that trips the breaker, is having a problem, and is failing.
A motor that trips off from overload is on its way to failure if allowed to continue.
A motor that locks up and fails to start will blow the easily replaceable fuse, and not suffer any damage, and can live again.
Bob(iwire) You do fine on all levels.
The fuses are the most important protection for any motor that may lock up from loading problems.
A motor winding will smoke a lot before the branch circuit breaker wakes up.
The overload heaters remain cold even after the fire dept arrives.
A motor circuit that trips the breaker, is having a problem, and is failing.
A motor that trips off from overload is on its way to failure if allowed to continue.
A motor that locks up and fails to start will blow the easily replaceable fuse, and not suffer any damage, and can live again.
Ed MacLaren
Senior Member
Re: motor starter
"One-liners" that over-simplify a technical subject have some entertainment value, but do little to help educate those who have asked questions.
As some of the others have already stated, overload relays do a fine job of protecting motor windings against overheating damage due to overloading, when applied properly.
How many installers ever read the instructions for relay adjustment/heater selection, and in particular the fine print referring to motor service factor.
And then there are the maintenance people (and unfortunately, some electricians) who think that the remedy for a tripped overload relay is to jack up the setting dial, or install higher rated heaters.
When overload protection is correctly installed, the overcurrent device, whether it be fuse or breaker, only has to protect the complete circuit against short circuit/ground fault conditions. Either will do the job equally well if applied properly.
For "non-standard" installations, follow the motor manufacturers recommendations.
In many of the motor burn-out situations that I have investigated, the installation simply was not done properly.
svaurez - To answer your question, if the motor branch circuit is properly protected by the circuit breaker, the combination starter was not required, however, it may have been given to you because a disconnect was required at the starter location, due to it's location in relation to the breaker panel.
Ed
[ April 01, 2003, 07:15 PM: Message edited by: Ed MacLaren ]
"One-liners" that over-simplify a technical subject have some entertainment value, but do little to help educate those who have asked questions.
As some of the others have already stated, overload relays do a fine job of protecting motor windings against overheating damage due to overloading, when applied properly.
How many installers ever read the instructions for relay adjustment/heater selection, and in particular the fine print referring to motor service factor.
And then there are the maintenance people (and unfortunately, some electricians) who think that the remedy for a tripped overload relay is to jack up the setting dial, or install higher rated heaters.
When overload protection is correctly installed, the overcurrent device, whether it be fuse or breaker, only has to protect the complete circuit against short circuit/ground fault conditions. Either will do the job equally well if applied properly.
For "non-standard" installations, follow the motor manufacturers recommendations.
In many of the motor burn-out situations that I have investigated, the installation simply was not done properly.
svaurez - To answer your question, if the motor branch circuit is properly protected by the circuit breaker, the combination starter was not required, however, it may have been given to you because a disconnect was required at the starter location, due to it's location in relation to the breaker panel.
Ed
[ April 01, 2003, 07:15 PM: Message edited by: Ed MacLaren ]
Re: motor starter
I attempt to authenticate all of my responses by technical document substantiation.
NEMA Standard MG-1-1993 is the document of reference.
Locked rotor current is the most damaging occurrence for a motor. Only proper selected time/trip devices will provide this protection.
Fuses are the most reliable and cost effective device.
Branch circuit breakers and running overloads will not reliably provide this protection.
[ April 01, 2003, 10:08 PM: Message edited by: bennie ]
I attempt to authenticate all of my responses by technical document substantiation.
NEMA Standard MG-1-1993 is the document of reference.
Locked rotor current is the most damaging occurrence for a motor. Only proper selected time/trip devices will provide this protection.
Fuses are the most reliable and cost effective device.
Branch circuit breakers and running overloads will not reliably provide this protection.
[ April 01, 2003, 10:08 PM: Message edited by: bennie ]
Re: motor starter
Bennie, Being a believer in breakers that I am I strongly disapprove of a thermal mag. breaker as being a part of a combination starter as they may not provide the motor circuit protection that an MCP will. But because of their thermal (I2t) element they may be able to provide locked rotor protection if sized properly. However, the magnetic pickup setting of a TM breaker may not provide adequate motor circuit protection. I promote the mag. only MCP only if set correctly but they do not respond to the I2t function which results from a locked rotor condition. And, as such, it will not provide protection against a locked rotor condition. One would have to relay on properly sized heaters and the OLR for locked rotor protection which you stated wasn't acceptable or reliable per NEMA Standard MG-1-1993.
If so, then how would a fuse be selected the would be reliable in preventing damage to a motor in the event of a locked rotor condition without going to a solid state overload device?
Needless to say this is an interesting as well as important topic.
Bennie, Being a believer in breakers that I am I strongly disapprove of a thermal mag. breaker as being a part of a combination starter as they may not provide the motor circuit protection that an MCP will. But because of their thermal (I2t) element they may be able to provide locked rotor protection if sized properly. However, the magnetic pickup setting of a TM breaker may not provide adequate motor circuit protection. I promote the mag. only MCP only if set correctly but they do not respond to the I2t function which results from a locked rotor condition. And, as such, it will not provide protection against a locked rotor condition. One would have to relay on properly sized heaters and the OLR for locked rotor protection which you stated wasn't acceptable or reliable per NEMA Standard MG-1-1993.
If so, then how would a fuse be selected the would be reliable in preventing damage to a motor in the event of a locked rotor condition without going to a solid state overload device?
Needless to say this is an interesting as well as important topic.
- Location
- Wisconsin
- Occupation
- PE (Retired) - Power Systems
Re: motor starter
Yesterday, I attended a fuse seminar put on by Ferraz-Shawmut. They specifically recommended against using fuses to provide motor running protection.
In fact their fuse selection software choses fuses based on coordination with overload relays. Time current curves are included with the software. http://www.ferrazshawmut.com/resources/selectafuse.html
[ April 03, 2003, 11:01 AM: Message edited by: jim dungar ]
Yesterday, I attended a fuse seminar put on by Ferraz-Shawmut. They specifically recommended against using fuses to provide motor running protection.
In fact their fuse selection software choses fuses based on coordination with overload relays. Time current curves are included with the software. http://www.ferrazshawmut.com/resources/selectafuse.html
[ April 03, 2003, 11:01 AM: Message edited by: jim dungar ]
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