Fuses for control transformers?


Thread Starter

Mark Antony


1000VA transfomer.
Primary voltage: 400V
Secondary voltage: 110V (one leg earthed)

Now maybe I'm taking this too simply but:

ideally max. current on primary side:
1000VA/400V = 2.5A

So a 6 Amp fuse is more than enough from an overload point of view?

But the 6 amp fuse blows due to inrush current...
A 10 amp fuse blows due to inrush current...
A 16 amp (or was it a 20 amp cant remember?)was OK.

How does a 16 amp fuse protect transformer from overload at 400V, yet inrush current prevents anything smaller?
Fuses are a science among themselves...

Are you using 3 phase or single phase?

Three phase Power=V*I*Sqrt(3)

Single phase Power=V*I

The primary side of the xfrmr should have both legs fused at roughly 1.5 times max current. And the 'hot' leg of the secondary side should be fused for your application, but no more than 1.5 times max current on secondary.

The trick however is to use slow-blow type fuses like the Limitron series from Bussman. These fuses are meant for inductive loads (like xfrm's where there is a high inrush current). They will allow relatively slow peaking currents to pass, while blowing when a short circuits are detected. Best bet is to ask the vendor directly.

You must use the correct fuse type/class to protect your equipment.
Watch the type (Class of fuse too). The larger the control transformer the more important this gets. Example: 3KVA or bigger control transformer should be using a Class J (or better) fuse for inrush capability.