Transformer primary to secondary overcurrent

[dm9289]

I work in an industrial process with three different kettle height heating grids for melting zinc.
It has 480v that feeds an SCR which feeds a 480-240v transformer no secondary protection. Design is 300amp primary/ 570amp secondary.

Primary SCR fuse blows about once a day we suspect some zinc overflow may have caused a problem. This has been the case in the past. We cannot enter this area for several months due to process needs.

My understanding was the primary and the secondary are not electrically connected so a problem on secondary should not affect primary. For example shorted service drop will burn without blowing primary utility fuse.

Could someone help me understand the concept of why this scr fuse blows. Circuit pic for one grid attached

 

[don_resqcapt19]

I work in an industrial process with three different kettle height heating grids for melting zinc.
It has 480v that feeds an SCR which feeds a 480-240v transformer no secondary protection. Design is 300amp primary/ 570amp secondary.

Primary SCR fuse blows about once a day we suspect some zinc overflow may have caused a problem. This has been the case in the past. We cannot enter this area for several months due to process needs.

My understanding was the primary and the secondary are not electrically connected so a problem on secondary should not affect primary. For example shorted service drop will burn without blowing primary utility fuse.

Could someone help me understand the concept of why this scr fuse blows. Circuit pic for one grid attached

That is only because the utility sizes their primary fuse to prevent any fault on the load side of that fuse from causing the next upstream protective device from opening. The do not typically protect their transformer or the transformer secondary conductors.
The current on the secondary will be seen on the primary in proportion to the turns ratio. The SCR fuse is doing its job...protecting the SCR from excessive current that would damage it.

 

[dm9289]

That is only because the utility sizes their primary fuse to prevent any fault on the load side of that fuse from causing the next upstream protective device from opening. The do not typically protect their transformer or the transformer secondary conductors.
The current on the secondary will be seen on the primary in proportion to the turns ratio. The SCR fuse is doing its job...protecting the SCR from excessive current that would damage it.

Yep I think I put to much emphasis on power company angle

 

[TwoBlocked]

Lots of things could cause this on both the primary and secondary side. Loose connections can cause havoc with a controller trying to keep a set point with either the signal input or the power output coming and going. Since this is an intermittent problem, a good place to start is with loose connections.

But as for the idea of "...the primary and the secondary are not electrically connected so a problem on secondary should not affect primary." Well, since the secondary is not grounded (typical on an SCR controlled furnace) what if one leg of the secondary becomes solidly grounded? This would not cause a fault. But what if the other leg intermittently also becomes grounded? Yes, this would cause high current on the primary.

 

[dm9289]

Lots of things could cause this on both the primary and secondary side. Loose connections can cause havoc with a controller trying to keep a set point with either the signal input or the power output coming and going. Since this is an intermittent problem, a good place to start is with loose connections.

But as for the idea of "...the primary and the secondary are not electrically connected so a problem on secondary should not affect primary." Well, since the secondary is not grounded (typical on an SCR controlled furnace) what if one leg of the secondary becomes solidly grounded? This would not cause a fault. But what if the other leg intermittently also becomes grounded? Yes, this would cause high current on the primary.

Thank you

 

[synchro]

... Since this is an intermittent problem, a good place to start is with loose connections.

An intermittent connection in the feed to the transformer might cause frequent occurrences of inrush current, making it more likely to blow the fuse. Also, SCRs are semiconductors which need relatively fast fuses for protection compared to to that needed for protecting conductors.

 

[dm9289]

An intermittent connection in the feed to the transformer might cause frequent occurrences of inrush current, making it more likely to blow the fuse. Also, SCRs are semiconductors which need relatively fast fuses for protection compared to to that needed for protecting conductors.

Yep they are fast and expensive fuses. Oddly with our process we are pretty sure molten overflow is the problem but we can only access the area 1 time a year. Over that year many things build up. We are nearing that year now, last year same problem ended up debris related