From: tannersgreen@gmail.com   
      
   On Tuesday, June 23, 2015 at 11:23:54 PM UTC+1, Andrew Gabriel wrote:   
   > In article <23291c15-87e7-40f8-9eec-816033642df1@googlegroups.com>,   
   > 	Stephen Biddle  writes:   
   > > 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.   
   >    
   > 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?   
   >    
   > --    
   > Andrew Gabriel   
   > [email address is not usable -- followup in the newsgroup]   
      
   Thanks Andrew - very helpful   
      
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