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Why do some porcelain pots explode?
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D6130
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From what I remember working on the Southern Region in the 1970s and '80s, the most common cause of pots exploding was through current arcing to earth during heavy rain, causing the pots to heat-up rapidly. It was a particular problem close to level crossings during the thaw following a moderate to heavy snowfall, when salt from the road, mixed with melt water, would evaporate into a saline solution mist which would stick to the outside surface of the pots, making for an even greater risk of arcing.
eastwestdivide
Established Member
Back in the 80s, I experienced this at Eastleigh, during a terrific downpour. A really loud bang, I turned to look and saw a bright arc for a few seconds then a puff of smoke. Platform line taken out of use, and platform alterations for the rest of the day.
Deepgreen
Established Member
Moisture entering through minute cracks is most likely. It surprises me how infrequently direct (uninterrupted) rain runoff from the live rail to contact with earth seems to happen.
59CosG95
Established Member
The freeze-thaw expansion effect is more common on the porcelain insulators used above track level 
The connections between the insulator cores and the insulator end fittings (hooks, eyes, sockets etc.) are the likely weak point (this being an educated guess on my part) and stresses caused by abnormal mechanical loading probably lead to the cracks that pave the way for freeze-thaw.
There have been some instances too where some examples of pond-life who inexplicably had access to an air rifle would take pot-shots at the insulators! As a result of this, there's now a programme on certain routes to swap to polymeric insulators.
The connections between the insulator cores and the insulator end fittings (hooks, eyes, sockets etc.) are the likely weak point (this being an educated guess on my part) and stresses caused by abnormal mechanical loading probably lead to the cracks that pave the way for freeze-thaw.
There have been some instances too where some examples of pond-life who inexplicably had access to an air rifle would take pot-shots at the insulators! As a result of this, there's now a programme on certain routes to swap to polymeric insulators.
Dr Hoo
Established Member
I was surprised that this thread didn't generate a bigger response. Whilst freeze-thaw and tiny cracks might be a cause, in my experience it was 'dirty' pots, most commonly because of de-icing fluid (which is obviously a conducting medium). It was an especial problem in platforms where de-icers might stand whilst waiting to reverse, change crews and so on and a single pot might get heavily covered. Basically the electricity would establish a 'creepage path' over the pot, which could get hotter and hotter until it caused an explosion.
I was at a station one day, when I was a Southern Region manager. It wasn't raining or freezing; just an ordinary day. A pot was smoking gently as a short train (4-VEP, I think) drew in and stopped routinely a little short of the pot. Suddenly there was an explosion and the compressor on the unit died as the power tripped. In accordance with normal practice the Electrical Control Room did a re-charge and the unit kicked back into life. Unfortunately the pot (or more correctly the caked-on fluid) caught fire and set light to the large amount of litter that was in the 'four-foot' as common at many stations. I walked across the 'Electrification' direct phone, rang the controller (who knew who I was) and asked him to discharge the section again while I put the fire out. Rang the signalbox from the platform office and explained what was going on. Then grabbing a 'blue' (dry powder) fire extinguisher I went to the driver of the standing train, confirmed that the power was definitely off and he wouldn't be moving, jumped down onto the track and put out the fire. Climbed back onto the platform, rang the controller again, asked for a recharge, which held and didn't cause another fire, rang the signalbox, confirmed that the driver had power, brake pressure and so on and saw the train despatched. All done in about five minutes. Got back to my office, wrote up the Fire Report form, popped it into a despatch bag for Waterloo HQ, asked the station supervisor to order a replacement extinguisher and thought nothing more of it.
A couple of days later I was 'honoured' by a visit from the Chief Regional Fire Inspector. However, this wasn't to congratulate me for prompt action.
"Do you know how much refills for dry powder extinguishers are?"
"No, but I've got an idea that you're going to tell me."
"FORTY-SEVEN POUNDS!"
"So are you telling me that I shouldn't have used an extinguisher specially marked 'For Use On Electrical Fires' on an electrical fire?"
"Certainly not! There was no need. You should have smashed to pot out with a sledge hammer during the isolation and left the litter to go out of its own accord."
"Do you think I walk around with a sledge hammer stuffed inside my trousers in case a pot catches fire?"
The Chief Regional Fire Inspector returned to Waterloo, no doubt feeling that half a day of his time had been profitably employed.
I was at a station one day, when I was a Southern Region manager. It wasn't raining or freezing; just an ordinary day. A pot was smoking gently as a short train (4-VEP, I think) drew in and stopped routinely a little short of the pot. Suddenly there was an explosion and the compressor on the unit died as the power tripped. In accordance with normal practice the Electrical Control Room did a re-charge and the unit kicked back into life. Unfortunately the pot (or more correctly the caked-on fluid) caught fire and set light to the large amount of litter that was in the 'four-foot' as common at many stations. I walked across the 'Electrification' direct phone, rang the controller (who knew who I was) and asked him to discharge the section again while I put the fire out. Rang the signalbox from the platform office and explained what was going on. Then grabbing a 'blue' (dry powder) fire extinguisher I went to the driver of the standing train, confirmed that the power was definitely off and he wouldn't be moving, jumped down onto the track and put out the fire. Climbed back onto the platform, rang the controller again, asked for a recharge, which held and didn't cause another fire, rang the signalbox, confirmed that the driver had power, brake pressure and so on and saw the train despatched. All done in about five minutes. Got back to my office, wrote up the Fire Report form, popped it into a despatch bag for Waterloo HQ, asked the station supervisor to order a replacement extinguisher and thought nothing more of it.
A couple of days later I was 'honoured' by a visit from the Chief Regional Fire Inspector. However, this wasn't to congratulate me for prompt action.
"Do you know how much refills for dry powder extinguishers are?"
"No, but I've got an idea that you're going to tell me."
"FORTY-SEVEN POUNDS!"
"So are you telling me that I shouldn't have used an extinguisher specially marked 'For Use On Electrical Fires' on an electrical fire?"
"Certainly not! There was no need. You should have smashed to pot out with a sledge hammer during the isolation and left the litter to go out of its own accord."
"Do you think I walk around with a sledge hammer stuffed inside my trousers in case a pot catches fire?"
The Chief Regional Fire Inspector returned to Waterloo, no doubt feeling that half a day of his time had been profitably employed.
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