Clamp On Heater Failures/ Testing

[Flux70]

I am new here. I have read the rules and have been in the controls industry for the past 15 years. I have learned a lot along the way. Most of my work has been in machine integration/ automation/ process engineering. I love what I do.

With that said I am a little stumped on something. I have a small stainless water tank which has 8 each Watlow strip type clamp on heaters installed. Each one is around 6" in length rated 240VAC/150W. All wired in parallel.

I am trying to get a hold of Watlow to find out if a resistance reading from the wiring terminal to the case of 5 megaohms is normal. As the 30mA ABB resistive current device is tripping. I don't have any new ones to test. Thought I might post this up here and see if anyone has experience with this.

This all came about after the tank was installed for about two minutes then tripped the RCD. Brought it back to the shop, tore down insulation, and found one heaters terminals loose. Interestingly enough, when testing that heater it shows 0 ohms from wiring terminal to its case. all others show 5 megaohms.

Any insight greatly appreciated.

 

[GoldDigger]

Eight units at 5 megohms and 120V will only amount to less than 1ma of current and should not trip your RCD.
Chances are the one with the zero ohm measurement is your problem.

 

[Flux70]

Problem solved. The heaters are constructed of Magnesium Oxide and are therefore hygroscopic. This means when new a reading of around 25-30 megohms is normal along with some leakage current. Once powered up and the moisture is baked out the resistive value increases into the gig range. I bypassed the RCD and ran the heaters for 4 hours. All OK.