From: andrew@cucumber.demon.co.uk   
      
   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]   
      
   --- SoupGate-Win32 v1.05   
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