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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

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  • \$\begingroup\$ 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? \$\endgroup\$ – Bimpelrekkie May 26 '20 at 11:53
  • \$\begingroup\$ 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. \$\endgroup\$ – Mohit May 26 '20 at 12:03
  • \$\begingroup\$ 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? \$\endgroup\$ – vu2nan May 26 '20 at 13:22
  • \$\begingroup\$ The transformer take either 208V or 240V and the inputs are labeled. The o/p is 24V 40VA. I provided input to the 240V \$\endgroup\$ – Mohit May 26 '20 at 14:34
  • \$\begingroup\$ 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! \$\endgroup\$ – Henning Larsen Feb 23 at 11:38
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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.

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  • \$\begingroup\$ 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. \$\endgroup\$ – Mohit May 27 '20 at 2:17
  • \$\begingroup\$ 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 \$\endgroup\$ – Kyle B May 27 '20 at 5:45
  • \$\begingroup\$ 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. \$\endgroup\$ – Kyle B May 27 '20 at 6:21

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