The current UK regs don't insist on it. Can you actually get to it when the loco is fully assembled and running to change it? I haven't come across one yet and I wouldn't fit one myself unless I had to.

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Can anyone reference the "recently added requirement" for CE marking? I can think of a few reasons why there is not much point in a 5" gauge boiler. One being where would you get a plug of known composition / quality in that size that you would be confident of fusing? Do those of you that have them fitted remove them on an annual basis for inspection? If you drop a plug you are supposed to have a re-inspection of the boiler. I suspect the requirement has been added by the certifying body that underpins the CE process rather than any actual change in standards but happy to be proved wrong.

Paul.

Hi Paul, I can only say what I was told, I'm sure that professional boiler makers wouldn't fit one unless forced too to have the then all important CE stamp. As for the plug, you make this yourself, make a blanking plug to fit, drill a suitable hole through the centre and soft solder a copper rivet to it with the head inside the firebox. The idea being that the rivet head will protect the solder for normal running but if too hot the solder will melt thus extinguising the fire. If the plug drops you'll need to remove the boiler anyway and thus it will need testing.

Pete

In the many, many years of model loco operation within the ME hobby, how many instances of boiler damage due to low water level have there been? I've not heard of one.

I find it hard to visualise a small fire, say 0.5kg of coal, releasing around a maximum of 1-2kW or so of heat, significantly damaging a typical silver soldered copper boiler, which will contain at least, some water. When I've soldered a boiler I might use heating of up to 20kW with plenty of firebrick insulation and, even then, it's a stuggle.

Just looked at the current version of AMBSC, para 5.7 which states that a fusible plug SHALL be fitted and makes no concession for diameter, which means it is mandatory on all boilers. It also states the fusible material shall be commercially pure tin, I read this meaning solder with at least 99% tin (eg lead free solder) and leaded solder if you still have some not being allowed.

Lets face it a train is unlikely to encounter a hill sufficent to uncover the crown for long enough to do damage or wake up the plug.

Road steam is another matter and over 3" models might be long enough to expose the crown on a hill, no doubt priming like crazy at the same time making the issue even worse.

All that said on a train it's fit and forget.

With respect Peter, it is not about melting copper, but melting the silver soldered joints the boiler is made with, around 700C should do the trick if held for a little time.

That said, I prefer not to have a fusible plug fitted, preferring to blank off the bush, since I have in the past had several plugs fail for no good reason.

Also in larger locos with fitted ashpans, the boiler has to be removed to replace the plug, bit annoying if the plug fails for no good reason.

Regards

Brian B

Forgot to add, thanks for sharing your own experience with fuseable plugs, Brian. Paul Tompkins did say to me that he wouldn't advise fitting a blank for the reasons you give.

Regards

Pete

This sounds like mission creep! It is known that Winson Brits have no water above the crown if the water is in the bottom of the glass**, but we've got 2 in our club with no bother. If you have a fusible plug in a full size boiler it has to be inspected at regular intervals to make sure it isn't all sealed up with scale. This would be a boiler off job on many models. How many silver soldered boilers have actually failed due to low water? We've had one soft soldered one, but no silver soldered as far as I know.

** correspondence in ME some time ago

Some interesting views here both for and against. One contributor states you make your own plug with soft solder and a rivet (a practice historically advocated by various ME "guru's). The American boiler code quoted by another requires pure tin – sensible standard depending on the pressure of the boiler. Tin melts at around 232dgrees C. Temperature of saturated steam is around 177 degrees C at 100psi, so on a boiler of WP pressure of 100psi or less you have a reasonable margin although the tin probably will not need to fully melt in order for the plug to fail so it may relieve at a lower temperature than the actual melting point of tin! If you use pure lead with a meting point of around 328 degrees C you have a better margin for service and unwanted false alarms but the strength of copper decreases to around 95% at 100 degrees and further decreases to around 90% at 300 degrees (graphs on interweb I consulted have poor definition so very course values). If you alloy tin and lead the melting temperature can in fact be increased above the constituents which makes it worse! So if you are making your own plugs you need to very sure of the soft solder you are using to end up with a product that delivers the required protection and does not give a false sense of security!

Then we have the maintenance aspect. The plug itself as also mentioned in this thread needs to be removed, inspected and cleaned on at least an annual basis, on full size loco's they are inspected at every wash out (interval between 14 and 30 days steaming). If a layer of scale deposits on the water side of the plug this will reduce the heat conduction away from it and lead to premature melting, the scale can also block the passage left by the melted material and stop the discharge!

It was mentioned an accident resulted from a dropped plug through fire being blown out of the door! To paraphrase a famous film "it's not supposed to blow the bloody doors off". Clearly in that case the bore of the plug was far too large. The purpose of the plug is to give a warning it is not supposed to act as a safety valve to dump the boiler pressure into the fire!

Summary fusible plugs are no substitute for proper boiler management! They are difficult to make and size to be effective. If not regularly maintained they can be unreliable. In anything less than a large 4" scale TE or larger 7 1/4" loco boiler they are unlikely to be of any real practical benefit. A safety device you cannot rely on is not worth having.

I have previously tried to find reliable documented accounts of small boiler failures per se let alone failures due to low water level and found nothing. There is a good ORR report on a narrow gauge loco the driver of which was inexperienced and if I am thinking of the right one managed to continue to run the boiler for a time with a dropped plug and completely wrecked the boiler, plug didn't save anything there! So back to the previous point, manage your boiler competently if you want to prevent damage! Definitely don't rely on something cobbled together in your shed with unknown or unverified materials!

Paul.