Overload, short circuit ground fault protection

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Overload protection is protection designed to address motor overload caused by such thing as mechanical failure, changes in loads etc. It is most often provided by internal protection "motor is thermally protected" or by overload devices such as "heaters" or electronic OL units. It can be provided by fuse(s)
See 430 Part III (430.32)

SCGF protection is designed to protect the wiring, motor and controller in case of faults, phase to phase or phase to ground.
Most often provided by circuit breakers or fuses which are sized to allow motor start-up loads.
See 430 Part IV (430.52)

In some cases fuses can be used to be both OL and SCGF protection. (see 430.55)
 
greenawaltmark4 said:
When sizing for a motor why do we use fuses and breaker to protect

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maybe, because typically a motor has a disco nearby if the motor does not have self protection, be it blade/button/lever style, and those items, in their fused format, will use std size fuses, because installing a std breaker panel (like a "spa" panel) just isnt cost effective, and although std breakers can act as a disco, they are not meant to cycle like a disco. a fused disco on a thermally protected device is still ok.

so thats the fuse part.


breakers - typically to protect the wiring of that ckt at the "main"/source panel, etc. these are typically of the resettable type, and are made in various formats, trip curves, and types.

fuses can still be found in main panels :(

as just one of many examples:


fuse2.jpg
 
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greenawaltmark4 said:
I'm installing power for a 15 hp motor for a compactor , the specs call it out to have a fused disconnect

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That is design choice by the mfg. You most likely have a motor controller with overloads as well. The complete assembly was tested by UL with a certain size and type of fuses ahead of it. The mfg could have had it tested with/or a CB. Design choice, as long as UL is happy with the test results.
 
greenawaltmark4 said:
I'm installing power for a 15 hp motor for a compactor , the specs call it out to have a fused disconnect
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Regarding motors, for most motors, overload protection and short circuit/ground-fault protection are separate handled by 'devices'. Small motors, at or under 15A, the overload protection is built into the motor or integral control. Over 15A some have integral overload, but most do not. This is why many require motor starters, as the overload protection is part of the starter (considered a controller under Code).

Overload protection not only helps protect the motor from overheating, it also protects the motor supply conductors from overcurrent. With general circuits, the fuses and breakers handle overcurrent. Becuase motors are required to have overload protection, all that is required for the supply conductor protection at the point of supply is protection against short circuits and ground faults.

If your specs say a fused disconnect is required, you cannot use a circuit breaker in its stead. Are their any other details associated with this spec that you are leaving out? It seems you are feeding us details one at a time... :D
 
Because of the nature of how motors work, many typically have a high surge of current at startup. Your protective devices must be able to ride through this or they will trip every time the motor attempts to start. Typical fuses and breakers commonly used for other then fractional horsepower motors often will not provide low level overload protection and instantaneous short circuit/ground fault protection simultaneously at desired response levels for each type of protection and therefore it is more desirable to have fuses or breakers that will react rapidly to short circuits/ ground faults as well as a motor overload protection device that reacts slower to overcurrent but will react more in line with the heating characteristics of a motor that is only slightly overloaded. - A motor may need 6-8 times FLC for short circuit/GF protection to allow it to start, but also has a thermal overload that responds much slower and if it is loaded to say 130% FLC it won't trip instantly but over enough time it still trips to prevent thermal damage from occuring to the motor. This does require proper selection/setting of protection devices or else you may damage motor anyhow if it is overloaded.
 
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