Coordinating fuses with molded case breakers

[mbrooke]

I'm afraid to ask. But how easy is it to coordinate fuses with non adjustable molded case circuit breakers? Doable? Fuse to fuse is worked out- it was easy.

Any experience on how or what fuses have worked for you?

I have panel boards with 125 amp and below THHQL, TEY and QOB branch circuit breakers. I'd like it so a fault on any branch does not take out the feeder.

 

[dkidd]

See page 19.

If a fuse is upstream and a circuit breaker is downstream, at some point the
fuse time-current characteristic crosses the circuit breaker time-current
characteristic. For short-circuit currents at that cross-over point and higher,
the upstream fuse is not coordinated with the down stream circuit breaker.
Figure 29 shows a 400A fuse with downstream 100A circuit breaker.
Coordination is not possible above approximately 5,000 amps as shown in
the overlap of the time-current curves (the current axis is 10x).

http://www1.cooperbussmann.com/pdf/b00ff3e7-c335-480f-a847-1696a3f99974.pdf

 

[mbrooke]

See page 19.

If a fuse is upstream and a circuit breaker is downstream, at some point the
fuse time-current characteristic crosses the circuit breaker time-current
characteristic. For short-circuit currents at that cross-over point and higher,
the upstream fuse is not coordinated with the down stream circuit breaker.
Figure 29 shows a 400A fuse with downstream 100A circuit breaker.
Coordination is not possible above approximately 5,000 amps as shown in
the overlap of the time-current curves (the current axis is 10x).

http://www1.cooperbussmann.com/pdf/b00ff3e7-c335-480f-a847-1696a3f99974.pdf

Beautiful paper.


At first glance it looks like I'm quite limited in options.


Biggest factor, as I suspected, will be the short circuit values at the panelboards and load centers themselves.


Tough part will be getting data. I've never modeled resistance, reactance and susceptance of THHN in conduit, MC cable and switchgear. Z=R+jX...

 

[wbdvt]

Beautiful paper.


At first glance it looks like I'm quite limited in options.


Biggest factor, as I suspected, will be the short circuit values at the panelboards and load centers themselves.


Tough part will be getting data. I've never modeled resistance, reactance and susceptance of THHN in conduit, MC cable and switchgear. Z=R+jX...

You will have to model the system starting with the utility available fault current to determine what your fault current will be. You may find coordination not possible for your fault currents.

 

[mbrooke]

You will have to model the system starting with the utility available fault current to determine what your fault current will be. You may find coordination not possible for your fault currents.

Modelling is going to be the hard part. Though the back of the NEC's voltage drop tables might be good enough.

Wish there was a list like this from all cable manufacturers as well as one for conduit with various conductor setups (ie paralleled):

https://www.rm-electrical.com/wp-content/uploads/2018/10/SWA-Chart-RM-Electrical.pdf

 

[paulengr]

With thermal magnetic MCCB's you will ALWAYS have this problem. Think about it...the fuse is using an electrochemical reaction. The higher the current, the faster it goes so at the highest point on the fuse curve it is tripping in under a quarter cycle. The breaker is a mechanical action. Even if we have super fast microprocessor based detection of the onset of a short circuit and can manage to recognize and send a trip signal at fuse speeds (under a quarter cycle), we still have some time required for mechanical motion to open the breaker. The fastest, smaller thermal-magnetic breakers operate in about 1 cycle which is still 2-4 times slower than the fuse in the short circuit region.

The only way this ever coordinates is if the point where the fuse and breaker curves cross is above the available short circuit current.

Also your modelling effort to calculate available short circuit current is going to run into all kinds of problems. For instance how are you going to account for the residual energy from a motor load? The old Bussmann book actually does have decent instructions on performing the coordination study calculations (available short circuit current) by hand. But realistically except for very simple systems power system analysis software is the way to go. It's not enough just to model the impedance of the upstream path from the voltage source(s).

 

[mbrooke]

With thermal magnetic MCCB's you will ALWAYS have this problem. Think about it...the fuse is using an electrochemical reaction. The higher the current, the faster it goes so at the highest point on the fuse curve it is tripping in under a quarter cycle. The breaker is a mechanical action. Even if we have super fast microprocessor based detection of the onset of a short circuit and can manage to recognize and send a trip signal at fuse speeds (under a quarter cycle), we still have some time required for mechanical motion to open the breaker. The fastest, smaller thermal-magnetic breakers operate in about 1 cycle which is still 2-4 times slower than the fuse in the short circuit region.

The only way this ever coordinates is if the point where the fuse and breaker curves cross is above the available short circuit current.

Also your modelling effort to calculate available short circuit current is going to run into all kinds of problems. For instance how are you going to account for the residual energy from a motor load? The old Bussmann book actually does have decent instructions on performing the coordination study calculations (available short circuit current) by hand. But realistically except for very simple systems power system analysis software is the way to go. It's not enough just to model the impedance of the upstream path from the voltage source(s).

No disagreement here. Very well articulated, and rather succinctly.


What do you think about this program? Does it have fuse curves?


https://w3.usa.siemens.com/powerdis...ftware/pages/time-current-curve-software.aspx

 

[dkidd]

Last time I tried it it wouldn't work. I think it only worked for their breakers.

 

[mbrooke]

I believe you. I wish S&C would make an LV version of their coordinate.

 

[mbrooke]

With thermal magnetic MCCB's you will ALWAYS have this problem. Think about it...the fuse is using an electrochemical reaction. The higher the current, the faster it goes so at the highest point on the fuse curve it is tripping in under a quarter cycle. The breaker is a mechanical action. Even if we have super fast microprocessor based detection of the onset of a short circuit and can manage to recognize and send a trip signal at fuse speeds (under a quarter cycle), we still have some time required for mechanical motion to open the breaker. The fastest, smaller thermal-magnetic breakers operate in about 1 cycle which is still 2-4 times slower than the fuse in the short circuit region.

The only way this ever coordinates is if the point where the fuse and breaker curves cross is above the available short circuit current.

Also your modelling effort to calculate available short circuit current is going to run into all kinds of problems. For instance how are you going to account for the residual energy from a motor load? The old Bussmann book actually does have decent instructions on performing the coordination study calculations (available short circuit current) by hand. But realistically except for very simple systems power system analysis software is the way to go. It's not enough just to model the impedance of the upstream path from the voltage source(s).

Any idea at which point a 20amp power-r-line breaker will coordinate with a fuse?

Something tells me that over 1000amps, a 100amp RK low peak fuse will blow as fast as a 20amp MCB.

I'd use these, but they are unjustly expensive:

https://www.amazon.com/Cooper-Bussm...sr_1_2?keywords=tcf15rn&qid=1572337397&sr=8-2