DC Leakage - Page 1
Forum section: electronics / projects

author: 741
author_rank: Frequent Contributor
timestamp: January 17, 2020, 12:11:03 pm
content: In general, how is DC leakage measured in mains powered devices? (It can adversely affect AC leakage detection eg at the consumer unit).Say a mains-powered design hasSomething like a 'capacitor drop' PSUAlso a mains transformer with multiple independent secondariesBoth of the above then power some DC circuitry.Do I measure Iin, Iout for each PSU and somehow add to ensure under the 6mA limit that seems to be the accepted value?

author: Lesolee
author_rank: Regular Contributor
timestamp: January 18, 2020, 09:59:14 am
content: "DC leakage" is probably not the right description. It suggests connecting Line and Neutral together and applying 500 V relative to earth using a Megger or a PAT tester.It sounds like you are talking about measuring a steady DC Line-to-Neutral current. I would think a moving coil ammeter in series with the Neutral is the way to go. Presumably the AC current in such a circuit is pretty low. It is not clear (to me) what standard should be applied in this case.

author: 741
author_rank: Frequent Contributor
timestamp: January 20, 2020, 03:54:20 pm
content: From "Everything you wanted to know about Type B residual current circuit breakers but never dared to ask" by ABB.Quote

author: Lesolee
author_rank: Regular Contributor
timestamp: January 20, 2020, 04:40:37 pm
content: Quote from: 741 on January 17, 2020, 12:11:03 pmIt's going back 40 years now, but I was involved with a resistive-dropper powered circuit for an electric cooker (hot hob warning). But I also investigated a capacitive dropper version (which I don't think ever went into production).The point of interest is that if you put the capacitor into a zener so you get asymmetric drops, you can get a half-wave rectified low voltage, low current (but rather lethally live) DC. But the capacitor can't pass DC, so it can't put DC into the mains. At the time (I don't know about now) you could get away with injecting harmonics if the power drawn was small (<50W?).Different types of RCD are outside my comfort zone, but 6mA DC into a 30mA ac RCD seems pretty unpleasant.

author: 741
author_rank: Frequent Contributor
timestamp: January 21, 2020, 10:43:03 am
content: As I understand it then, the issue at the RCD is thateven thoughthe mains supply is AC, I could apply a pulsed load. If (for instance) this load is always near the + peak, the effect is a "nearly DC" current. This DC bias current can saturate the RCD coil.To measure this effect in my own device then, I'd have to provide equivalent response to a type 'B' RCD. The type B devices work by measuring the effect of DC bias on the inductance of a supplementary sense coil. It might be the only sensible option is to copy this technique (?).Given the existence of type B RCDs, why do some end-user devices on the market bother to duplicate that action? Is it primarily to provide a more graceful failure? Or is the on-board 'DC leakage' going to be certified as safely protecting a type A RCD from saturation effects?

author: Lesolee
author_rank: Regular Contributor
timestamp: January 21, 2020, 12:06:02 pm
content: Sadly I can't answer any of your questions.I do recall there being something called "burst-firing controllers".https://en.wikipedia.org/wiki/Zero_crossing_controlThese efficiently controlled power, but they and the thyristor angle controllers were all phased out due to the harmonic standards that became mandatory. I can imagine that there is a standard that relates to mains harmonics and DC current, updated to reflect the widespread use of RCDs in the home due to the updated wiring regs.It sounds as though you are not working on domestic supplies, but it might be helpful to phone up you local EMC test house and see if they can offer any advice.