# Fuse on Primary Side of Transformer keeps blowing

I have a HVAC which is having a 24V 40VAC transformer. I might have shorted the output of the transformer while working on the wires.

I noticed that the fuses on the input side of transformer blew off and I replaced them with time delay 2A fuses. I unloaded the output of transformer (open circuit) to test if transformer is providing the right volts. Here the fuses blew off again.

Is it because the transformer is bad or I am using wrong fuses here ?

Transformer: Input: 240V Output: 24V 40VAC

• I might have sorted the output Do you mean shorted? I unloaded the o/p of transformer what does o/p mean? If you want help, at least make your question clear and don't use abbreviations. Did you replace the fuse with a new fuse having the same ratings? – Bimpelrekkie May 26 '20 at 11:53
• It had mda 2A fuse which i replaced with mdl 2a fuse. The both are slow blow fuse but not sure if that's what is causing it. – Mohit May 26 '20 at 12:03
• Does the transformer name plate read 'Primary 240V~ 50Hz, Secondary 24V - 40VA'? Does it have only 2 windings and have you inadvertently applied 240V on the secondary side? – vu2nan May 26 '20 at 13:22
• The transformer take either 208V or 240V and the inputs are labeled. The o/p is 24V 40VA. I provided input to the 240V – Mohit May 26 '20 at 14:34
• If disrupting the current in a transformer you will generate voltage spikes. U=L*dI/dt. Så large/fast current change dI in short time dt will give a large voltage spike. L is the inductance of the windings. This could have damaged the isolation in your transformer and led to a short circuit. This you can measure with an ohm meter provided you know what value is should be! – Henning Larsen Feb 23 at 11:38

## 1 Answer

If the input side of the X-former is blowing fuses with no-load on the output, you killed your transformer when you shorted it before. In a no-load situations, using a previously known good transformer, the fuses (even if not the right size) should not be blowing.

• is there a possibility of inrush current with no load on secondary causes fuses to blew quickly? trying to understand how i can properly test the transformer to make sure its not bad. – Mohit May 27 '20 at 2:17
• No. An open secondary will result in nearly zero inrush current. Re: testing - You're already testing it with fuses ;) No point in me typing out "how to test a transformer", when you can google it... i.e. hunker.com/13414369/how-to-test-a-24-volt-transformer – Kyle B May 27 '20 at 5:45
• Actually that was a lame link. Sorry. youtube.com/watch?v=D0OQvuux6CE is a little better. Unfortunately, you can't always prove a transformer is good with just an ohmmeter, but you can sometimes prove it's bad. – Kyle B May 27 '20 at 6:21