RCD's and stray capacitance to ground 1

[mjevans]

Hello out there. Can anyone help with this problem. I have a bank of resistive heater elements that by design also present a capacitive load to earth, protected by a 30mA RCD (ELCB). Here is my question, I assume that the current in the neutral is the resistive current only, whilst the line current will be the vector sum of the resistive and capacitive currents. The RCD will trip on the difference of these two currents. Is this the case? For the circuit outlined above, I have measured Ires = 2.4A and Icap =0.188A (I have also read that a 30mA device can trip at much lower currents, is the rated figure the worst case scenario?). I look forward to your comments!

 

[lyledunn]

mjevans,
I am not entirely sure what you mean, but many RCDs are manufactured to IEC standards such that they may operate within 50% to 100% their nominal values. Thus a 30ma RCD could operate at a current in excess of 15ma.

Regards,

Lyledunn

 

[mjevans]

Just clarify what I was asking. The resultant current would be 2.407A. Would the imbalance seen by the RCD therefore be 7mA and not the 188mA capacitive current?
thank you all for your help

 

[ScottyUK]

mjevans,

I think you are saying that the capacitance causes a leakage current from the line conductor to ground of some 188mA, which does not flow in the neutral conductor.

The RCD uses a core-balance in which the line and neutral currents should have a vector sum of zero. The 188mA capacitive current will be measured as 188mA, not as 7mA using the scalar quantities.

It is probable that this heater would trip your RCD.