Cartridge heater earth leakage at elevated temperature

[asimpson]

I am doing some R&D work by heating steel to 800C using high temperature cartridge heaters. I am getting some earth leakage around 3mA at 230V AC at high temperatures causing RCD to trip out.

I am in discussion with cartridge heater manufacturer to improve performance of their heaters.

Would there be any safe way of stopping the tripping short of de-rating RCD to higher leakage current.

Measured leakage rises sharply around working temperature and falls when temperature is lowered. I assume insulation in heater is not up to the job.

Thanks

 

[waross]

A fall in resistance of Magnesium-oxide (MgO) is common at elevated temperatures. When using heat to dry the ends of mineral insulated cable The ends must be allowed to cool before a good megger reading will be obtained.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Since you have a high current closely associated with metal you can have inductive or capacitive leakage which would have nothing to do with "insulation" causing your leakage current.

Where's this 3mA you're measuring?

T'wer me I'd try to confirm this hypothesis as then you could come up with an appropriate solution.

Possibly run the heater with DC or use an isolation transformer. Or assess the situation and use an RCD with a higher setting or dump the RDC altogether.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Measured leakage rises sharply around working temperature and falls when temperature is lowered.(800 deg. C)
Typical of MgO insulation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[7anoter4]

I agree with waross as MgO resistance is low for an elevated temperature and I agree with itsmoked that the cable capacity may add a few "mas" also. But my opinion is that a RCD [even of high sensivity] don't need to trip at 3 ma.[the usually rated is 30 ma and will trip instantaneously
at 20 ma].I think you have to check the RCD device. High sensivity RCD [in Europe, at least], is rated for 6,10 and 30 ma. There are RCD devices with variable sensitivity also.