Fuse and Breaker speed

[SilverArc]

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

 

[Zogzog]

Fuse clearing times are typically faster than breakers. Current limiting fuses can operate as fast as 1/4 cycle while standard fuses are around 3-4 cycles. A CB with INST protection is around 6 cycles without an INST trip could be 30 Cycles or longer.

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.

 

[dpc]

The speed of operation depends on the magnitude of fault current. If you look at the time-current curve for a current-limiting fuse, it appears as a nearly vertical line. For very high levels of fault current, it operates within a 1/4 cycle or so - it has to do this to be considered current-limiting. But at lower levels of fault current, it is much slower (and not current-limiting).

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.

 

[l3city]

good comment "dpc".

 

[tulum]

I was at a seminar put on by Schneider this past fall (florida, was pretty nice!).

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

 

[unclebob]

If the fuses are the only protective devices, if you lose one phase, what is going to open the other two? A motor running with only one phase don't run for long.

 

[alehman]

Modern MCCB's will open in less than 1/4 cycle, similar to fuses, under high fault current.

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.

 

[davidbeach]

Fuses have their uses, but that is far less than the fuse manufacturers would have you believe. Over time my take on fuses has become that they are useful, in series with circuit breakers, (I'm a big power guy by the way) where very fast clearing of bolted faults is desirable and the time necessary to replace the fuse is negligible compared to fix what ever it was that caused the fault to operate. The circuit breaker and its associated relaying need voltage or negative sequence over current to trip the breaker on loss of a fuse. Fuses can certainly be faster than power circuit breakers when the fuse is in, or very near, it current limiting range. Otherwise it can be very, very slow. Beyond the DC control power I'd never go for just a fuse alone and most applications can deal with 4-6 cycle clearing times of a properly set relay and associated breaker.

 

[LionelHutz]

There is no set answer. It depends on which fuse and breaker you are comparing and the fault current you expect. You could even compare a certain fuse and breaker and find the device that clears first changes depending on the fault current.