[Boatanchors] Fuses to protect transformers

[Brian Clarke]

Hello Wayne,
As you are trying to protect the transformer, it would be better to insert a 
soft-start circuit.
Fuses are designed to prevent fires. They are usually placed in the primary 
circuit to protect your household's fixed mains wiring.
Fast-blow fuses are not fast enough to protect electronics equipment. Have a 
look at any fuse manufacturer's charts of opening time vs current demand vs 
current rating vs ambient temperature.
It is always wise to use a fuse whose Voltage rating exceeds the maximum 
Voltage peak the fuse is ever likely to experience under fault conditions. 
You need to define the fault conditions. This means that you need to analyse 
the complete circuit downstream from the transformer looking for fast-rise 
Voltage possibilities, eg, switched inductive circuits.
73 de Brian, VK2GCE.

----- Original Message ----- 
From: <hwhall at compuserve.com>
To: <milsurplus at mailman.qth.net>; <boatanchors at mailman.qth.net>
Sent: Thursday, March 13, 2014 3:27 PM
Subject: [Boatanchors] Fuses to protect transformers


> For this 40s vintage Link Trainer electronics I'm working on, I'm thinking 
> that it'd be a good idea to fuse not only the primary but the secondary HV 
> winding, since these transformers are not too easy to find anymore. I'm 
> thinking of pigtail fuses inserted in the wiring under the chassis, to 
> maintain appearances. But I'm not sure about what fuse selection would be 
> adequate protection.
>
> Would a 1A fast blow fuse be OK for the secondary that probably delivers 
> around 100ma DC? Or to put it another way, how much over the normal 
> current output should I select a fast blow fuse? The filters are dual 
> section choke input with 15 & 25 mfd caps. I haven't reached the point of 
> firing up the whole thing, so I have to guess for now at the current.
>
> The common fuses I have available are marked 250V and the HV is 450V. Can 
> they be relied on to open and not arc?
>
> Any advice? Thanks!!
>
> Wayne
> WB4OGM