Fuse and Breaker speed
Fuse and Breaker speed
(OP)
Hi,
In one of the posts here, some body talked about
Circuit breakers being faster in operation time than fuses.
An engineer visited our office today and I happened to discuss with him about their facility policy about using Disconnects and Breakers for motor starters.His argument was that they use a disconnect and a fuse instead of a MCP breaker becaucse a fuse is faster in operation than a breaker.
Could you please elaborate, if this argument is justified.
Thanks
In one of the posts here, some body talked about
Circuit breakers being faster in operation time than fuses.
An engineer visited our office today and I happened to discuss with him about their facility policy about using Disconnects and Breakers for motor starters.His argument was that they use a disconnect and a fuse instead of a MCP breaker becaucse a fuse is faster in operation than a breaker.
Could you please elaborate, if this argument is justified.
Thanks






RE: Fuse and Breaker speed
However, there is more to consider than just clearing times and there are solutions to the breaker clearing times, depending on the application. For example, ABB has a relay for arc flash that can cause a trip in 2.5mS.
RE: Fuse and Breaker speed
A molded case circuit breaker operating on its instantaneous element (or an MCP) is a little slower than the fuse, but not much.
The fuse guys love to tout the speed of operation, but for motor protection, most motor faults are not bolted faults - there is a fair amount of fault resistance. In these cases, the fuse will often be slower than an MCP in clearing the fault because the fuse is not operating in its current-limiting range.
RE: Fuse and Breaker speed
RE: Fuse and Breaker speed
The Schneider engineer displayed lab test results that showed that because of the dynamic impedance of a MCCB the theoretical belief that fuses operate faster than breakers was invalid. He went on to show quite lengthy how depending on how much fault current and where on the curve the fault actually occured it was an unpredictable race between the breaker and the fuse.
The conclusion of his presentation was that unless tested by the manufacturer series rating of a fuse with a breaker is not a good idea... (not really part of the original post but relevant non the less)
Regards,
TULUM
RE: Fuse and Breaker speed
RE: Fuse and Breaker speed
I think Zog may be thinking of power (air frame) circuit breakers. Those sorts of operating times used to be common. Some modern power breakers have sub-cycle operating times.
In NEC areas, it is not permitted to use an MCP in lieu of fuses unless it is part of an approved combination starter. It cannot be used as a stand-alone device, such as a disconnect. A thermal-magnetic breaker or molded-case switch would be permitted as a disconnecting means.
RE: Fuse and Breaker speed
RE: Fuse and Breaker speed