To fuse or not to fuse (on the secondary side of the transformer)?

[moamps]

I have one anecdote about fuses and NTC resistor in power supply.
I received an huge audiophile amplifier from a friend at a service with a fault that the mains fuse burns out when it is turned on. I checked everything and set the rated fuse and everything worked ok. What actually happened? The amplifier has a soft start and a NTC resistor connected in series with the mains transformer to avoid large inrush currents. When the amplifier is turned on, the NTC limits the current with its high resistance but it starts to heat up and its resistance decreases so that the transformer gets full voltage in a short time. The NTC remains active in the circuit but is hot and its resistance is low. What was the friend actually doing?
He tested the audibility of silver and gold fuses (expensive) by turning on an amplifier with a standard fuse, the amplifier would work for a while, then he would turn off the amplifier, replace the ordinary fuse with an audiophile one, and turn on the amplifier again. And then the audiophile fuse would burn out, simply because the NTC resistor failed to cool down for that short time, increase the resistance, and decrease the inrush current.

It was a rather expensive experiment.

From my experience the worst problem in general with power supplies is the short-term loss of mains voltage, which can cause a lot of damage especially on computers power supplies where ordinary fuses are not too helpful.

 

[trobbins]

I'm not across what silver/gold fuses are available, but I am aware of the performance of the most common 5x20mm 250VAC rated fuses that are compliant to IEC 60127-2 and UL 248-14 and used by most manufacturers and sold by most main stream suppliers - with the IEC type common in Europe and many other countries, and the UL type in USA. The IEC spec fuses have much better specified performance limits than the UL, and therefore by design allow a much better confidence in knowing what fault conditions they must blow for. Having specified performance limits, those fuses can be selected to not blow under given power-up and continuous current conditions, and to blow for given levels of fault current.

Aiming to use a fuse should be based on much more than having a box of available fuses, especially for secondary side fusing where the type and rating of fuse should imho be much more aligned and designed for the circuitry being protected.

Even mains side fuses can often be optimised for protection of secondary side faults, especially when the value/type of a mains side fuse has been chosen to meet product safety requirements for a range of countries (eg. using a 2A fuse when a 1A fuse may well be adequate for local conditions and the setup of equipment).