Circuit Protection and SCR Fuses

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NetNathan

Net is where I be and Nathan is me
Location
Corona, CA
Occupation
Mgr. Electrical and Controls Eng
I have a 200A load with a 250A CB and I wired it sized for 250A.
The SCR Fuses that came with the 240A rated SCR are rated 350A.
The SCR fuses are 1 cycle fast fuses.

It is still okay to use the 250A CB protection (load x 1.25) with 350A SCR fuses, because the SCR fuses are rated for short circuit protection (spikes) only?
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
210513-1229 EDT

NetNathan:

There are various kinds of fuses and circuit breakers. Ones based on something thermal can have that thermal characteristic controlled somewhat by the design of the device. So you can have fast, normal, and slow tripping characteristics.

Semiconductor devices can not tolerate over current very long because they are very small objects with little thermal mass, are operated at very high current densities near full rating, and thus heat very quickly. Thus, a very fast blow fuse is desired for protection. A large transformer has a lot of thermal mass, and can tolerate over current for a moderate time. Thus, a slow blow fuse may be appropriate for its protection.

If you have a fast blow fuse protecting some device, that will adequately protect the device, then you can put any other protective device in series and it won't change the protection of the fast blow device. Which devices trip first does not matter because the fast blow is selected to protect the semiconductor.

.
 

NetNathan

Net is where I be and Nathan is me
Location
Corona, CA
Occupation
Mgr. Electrical and Controls Eng
210513-1229 EDT

NetNathan:

There are various kinds of fuses and circuit breakers. Ones based on something thermal can have that thermal characteristic controlled somewhat by the design of the device. So you can have fast, normal, and slow tripping characteristics.

Semiconductor devices can not tolerate over current very long because they are very small objects with little thermal mass, are operated at very high current densities near full rating, and thus heat very quickly. Thus, a very fast blow fuse is desired for protection. A large transformer has a lot of thermal mass, and can tolerate over current for a moderate time. Thus, a slow blow fuse may be appropriate for its protection.

If you have a fast blow fuse protecting some device, that will adequately protect the device, then you can put any other protective device in series and it won't change the protection of the fast blow device. Which devices trip first does not matter because the fast blow is selected to protect the semiconductor.

.

I understand all you explained..
So then it is okay to use the protected 250A feeders through the 350A fast fuses which are only needed to protect the SCRs in case of fast spikes?
I do not see running 350A feeders from a 250A CB which protects the transformer (The SCR is controlling the transformer).
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
210514-2114 EDT

NetNathan:

Correct.

For long term overload size the slower breaker for your longer time needs, then size the wires to that lower rating breaker, or for voltage drop.

.
 
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