30 amp heat pump blows 800 amp fuse in meter stack

[FREEBALL]

Can anybody explain the reason a 30 amp heat pump that is protected by said 2 pole breaker and fed from a 125a subfeed, would blow the 800 amp littlefuse, when it shorts out. I have checked all connections and still I am baffled why the 30 or 125 wouldn't have tripped before this. Found the fault after isolating it but just cant believe this was the culprit. Id like to make sure this was the culprit and not another issue being masked.

 

[iwire]

The instantaneous trip of the breakers is slower than the fuse opening.

 

[ghostbuster]

The instantaneous trip of the breakers is slower than the fuse opening.

Have the breakers tested or replaced and have them set-up as per the co-ordination study

 

[iwire]

Have the breakers tested or replaced and have them set-up as per the co-ordination study

My bet is that there has not been a coordination study at all.

 

[FREEBALL]

where can I find the coordination study and thankyou for the response I just spoke with Littlefuse and she also said the breakers are most likely not in series with the main fuse.

 

[iwire]

where can I find the coordination study

You pay for an engineer to do a coordination study.

I just spoke with Littlefuse and she also said the breakers are most likely not in series with the main fuse.

But obviously she was mistaken right?

 

[kwired]

Though you wouldn't think it would happen that way, like iwire said trip curve of the fuse is obviously a faster trip time at the current involved then it was for the feeder and branch circuit devices.

Can you tell us exactly what fuse type is and the breaker types. If you tell us you have FPE or Zinsco breakers many will tell you the problem is obvious

 

[augie47]

Just a word of caution before you arbitrarily change any component. The faster acting fuses are often there to limit fault current and a different type fuse might compromise the system.

 

[GoldDigger]

Just a word of caution before you arbitrarily change any component. The faster acting fuses are often there to limit fault current and a different type fuse might compromise the system.

:thumbsup:
If this were a common occurrence (and not an equipment defect) you would probably find it less involved to add fast fuses to the 30A circuit than to change the time characteristics of the main.
Of course those fuses would still have to allow the motor starting surge, which would be a small change in current at the main and a proportionally bigger one at the branch.