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I'm trying to understand a technical detail about breakers and fused disconnects.

Multipole breakers come with a tie bar that connects all poles together. Causing them all to trip at once, if one pole is tripped on its own.

Not so much with fused disconnects. When a single fuse blows, its partners do not blow. There is no common trip to this method of overcurrent protection. There's gang operation of the contacts when a user operates the device, but no common trip for when fault current causes the opening.

My question is, is the common trip feature a requirement or a selective requirement? If so, how do I determine when it is required, therefore not allowing me to use breakers and fused disconnects interchangeably.
 

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Carultch said:
I'm trying to understand a technical detail about breakers and fused disconnects.

Multipole breakers come with a tie bar that connects all poles together. Causing them all to trip at once, if one pole is tripped on its own.

Not so much with fused disconnects. When a single fuse blows, its partners do not blow. There is no common trip to this method of overcurrent protection. There's gang operation of the contacts when a user operates the device, but no common trip for when fault current causes the opening.

My question is, is the common trip feature a requirement or a selective requirement? If so, how do I determine when it is required, therefore not allowing me to use breakers and fused disconnects interchangeably.
You're confusing OCP with a disconnect.
 

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first of all, read this (scroll down to #4) http://ecmweb.com/content/common-misunderstandings-overcurrent-protection

secondly, common trip protects things like motors from single phasing, whereas a fused disco or handle tied 3 pole would not protect it.

thirdly, I'm not an expert on the code. 450 (some transformers) and boilers require common trip by NEC, but I'm not sure what else is actually required, or just spec'd by engineers (or installed at random by gear distributers)
 

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Aside from the specific mention in the Code of needing common trip CBs cited above, the decision to use CBs or fuses is usually made at a system coordination level. Fuses generally provide higher levels of fault interrupt capability and the ability to current limit better than CBs. But that is not always necessary, so the risks (stated below) must be weighed against any perceived benefits.

The biggest risk of using fuses, BECAUSE it does not provide common trip of multi-pole circuits, is that of single phasing of 3 phase loads, primarily motors. Other less prominent risks of fuses (compared to CBs) are:

  • Greater exposure to Arc Flash hazards to electricians if the fuses must be replaced live.
  • Increased likelihood of being replaced with incorrect sizes
  • Risk of being replaced by pieces of pipe or bolts!
But if you NEED the benefit of fuses in a system and CBs are not going to get you there, you CAN add separate protection against single phasing by using phase monitor relays either tied into the control circuits of motor starters, or tied into trip coils of larger Bolted Pressure (Pringle) switches. There are also BFI (Blown Fuse Indicating) fuses on larger sizes that pop up a pin that hits a limit switch that can be tied into control systems as well.
 

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And there are shunt trip disconnects and bolted pressure switches that trip on a blown fuse.
 
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