Fire hazard query

[ex seismic]

In the recent thread about fire hazards mention is made of adding a thermal fuse to mains transformers. Given that such fuses have to go on the outside of transformers, ideally on the windings, what temperature value is suitable?

This is topical for me, a friend has two valve receivers which have "suddenly stopped working", the one currently on my bench has a burnt out primary. A thermal fuse would hopefully have prevented the loss of this transformer.

Both his two sets are military, in spacious but more or less sealed cases, i.e. no ventilation but plenty of air space.

Gordon

[wireful3]

I have just a general comment about the fire hazard.

I have never felt comfortable with modern items where OFF does not switch the mains supply. I know that transformers can be very efficient and there should be no problem in leaving the primary connected if the load is switched off. The small size and construction of some of the cheaper units does not instill confidence. This encouraged me to make a practice of connecting my bench to a single switched power socket so that I can disconnect everything on leaving.

Perhaps it is significant that equipment from the late 30s and 40s still switched the primary even when transformers were probably better made.

I think Gordon's experience shows why this might be the reason

[Skywave]

Gordon - I realise that this Thread refers to the addition of a thermal fuse to the outside of a transformer, but I have seen many a transformer where the thermal fuse is integrated into the windings. Could this not be the case with the transformer you have that has symptoms of an open-circuit primary?

Just a thought.

Al. / Skywave

[Skywave]

Originally Posted by wireful3

I have never felt comfortable with modern items where OFF does not switch the mains supply. I know that transformers can be very efficient and there should be no problem in leaving the primary connected if the load is switched off.

I whole-heartedly agree with you, re: your first sentence.

However, as for "there should be no problem . . . ." (second sentence), this is open to debate. Personally, I agree with you: a mains switch, preferably double-pole, that when in the OFF position disconnects incoming mains from ALL internal components, is my favoured method.

Al.

[ex seismic]

Skywave,

Good point, however the receiver in question is an R107 dating from the 1940s. I don't think internal thermal fuses were even thought of in those days. The smell of this one is very indicative that it has died! The primary is in fact short rather than open and it has also taken out the chokes in the L and N lines. All rather sad.

I also have an R107 and would like to prevent a similar occurrence, hence my query.

Gordon

[Lucien Nunes]

I prefer thermal fuses around 100 deg C for general protection, unless normal operating temp is especially high. Most traditional insulating materials (e.g. class A) will withstand this and certainly won't ignite, yet no ordinary mains transformer runs hot enough to boil water so there's little chance of nuisance failure. More recent devices that operate at higher temps such as motors with class F insulation (155 deg C) some of which are nonetheless becoming 'vintage' these days, would perhaps merit a higher temp fuse.

Of course this assumes that you can get the fuse into good thermal contact with the winding, preferably poked into the core window and taped-on or similar. If this is impossible, such as with a canned oil-filled transformer such as a Parmeko, then a lower rating of fuse will be needed to account for the difference between the winding temp and the fuse. 60-80 deg C might be a good range subject to knowing the air temp around the transformer in normal operation. Heat is conducted well through the terminals so direct connection might assist temperature tracking. Naturally you can't solder the thermal fuse on though!

Thermal fuses can be tucked into droppers too, to give the equivalent of the spring-loaded solder joint beloved of TV designers. Also with a bit of experiment they might be made to respond to heat from valves. IIRC the position of the slow-blow fuse in the EHT cage on a CTV25 was chosen to pick up radiant heat from the PL509 to encourage it to fail early if the valve was running hot.

Lucien