Moisture in Magnesium Oxide insulator

Hi All
I work with electric heaters and have a problem with moisture getting into the element, causing the insulation resistance to drop. The BS spec for im mersion heaters says it must be at least 2M ohm at 500V. If I power up the heater it can dry out and recover. Does anyone know the minimum insulatio n resistance that I need to turn it on without causing permanent damage. I t seems to me that a leakage current could cause "tracking" and make the co mponent useless.
Stephen

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Tracking that results in permanent damage is caused by the insulating material breaking down and forming a conductor (or at least a partial conductor), such as plastics breaking down and leaving a carbon track. Magnesium Oxide will not breakdown, so you shouldn't be able to damage it any further by turning it on. If there was lots of moisture in there, you might get an arc right across that would leave vaporised metal, but the heater has had it by that point anyway (the magnesium oxide should be under enough pressure to quench any such arc unless the case is burst).
I don't have the PAT test guidelines to hand at the moment, but they used to allow you to operate mineral insulated heaters to drive off condensation before performing the insulation test. I just can't recall if that's still in the current version of the IEEE in-service inspection and testing document.
Obviously, it's important to check the casing is properly earthed. If there's an RCD in the circuit, that may prevent you operating the heater to dry it, but if it's leaking somewhere over 15mA (as may trip a 30mA RCD), then it's probably got too much water in to be considered still OK.
I would also say if you have an earth leakage and are contemplating leaving the heater in service, you should inspect the surface for damage, as the usual cause nowadays is that a hot-spot has formed and punctured the case, so it may not last much longer anyway. Given that you have to take the heater out to do this, is it really worth putting an old one back in?

On Tuesday, June 23, 2015 at 11:23:54 PM UTC+1, Andrew Gabriel wrote:
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nto the element, causing the insulation resistance to drop. The BS spec fo r immersion heaters says it must be at least 2M ohm at 500V. If I power up the heater it can dry out and recover. Does anyone know the minimum insul ation resistance that I need to turn it on without causing permanent damage . It seems to me that a leakage current could cause "tracking" and make th e component useless.
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Thanks Andrew - very helpful

"Stephen Biddle" wrote in message
Hi All
I work with electric heaters and have a problem with moisture getting into the element, causing the insulation resistance to drop. The BS spec for immersion heaters says it must be at least 2M ohm at 500V. If I power up the heater it can dry out and recover. Does anyone know the minimum insulation resistance that I need to turn it on without causing permanent damage. It seems to me that a leakage current could cause "tracking" and make the component useless.
Stephen
I have noticed that many of the cheaper Asian built units had crap sealing on the ends of the elements. As said before, it's difficult to heat them up to dry out the insulation but once that is done resealing the element ends with hi-temp silicon or whatever is your pleasure, usually corrects the problem. Assuming you have meggered the repair (during and after) to check for element damage. Prior to heating dig out the old sealant with a scriber or whatever, this has to be done anyway and allows the moisture escape easier.