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Looking to understand a ramp test result

Alan B

I visited a customer today who had an RCD tripping after a water spill on a hot plate.

It was a single RCD protecting the entire installation, Hager type AC, been trying to persuade her to change the board for a few years.


The bit that I find strange is that the RCD was seeing 15.9mA of earth leakage, measured with clamp meter across tails. RCD also tripped in 133mS with 30mA test from a socket.

I then did a ramp test on both 180 and 0 degrees, from a socket with the installation powered, expecting a very low trip current due to existing high leakage, but it tripped at 21mA on both directions. Either I missed something or there is something I don't understand about the RCD testing setup? The one thing I can see with hindsight is that I didn't repeat the tests with the MCB's switched off.

So why would the ramp have gone so high before tripping? This is on a TT earth with Zs @ DB of 17.5 ohms. Currently installations insulation resistance is only 0.47 M Ohms ar 250V, line / neutral joined to earth. I hope this improves as the hob dries out.

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    It is not impossible, lets assume that most of the 15mA of standing leakage is capacitance L-E due to capacitors within mains filters in electronic loads plus  the natural capacitance of the wiring, which may also be on the high side, given it is a whole site figure.

    This current is at right angles to the applied voltage, or if you prefer, offset in time by a phase shift of 90 degrees.

    The insulation test tells us that less than 1mA is actually resistive leakage (0,25M ohms would do that.) so lets assume it is to all intents and purposes all capacitance.

    The ramp test however will be dropping in a binary weighted set of resistors or similar so the current it induces L-E will be in phase with the incoming voltage, and that stopped at just over 20mA.

    Adding the two, correctly, with a bit of pythagoras, we get Sqrt (20*20 +15*15 ) = sqrt (400+ 225)= sqrt (625) = 25mA at about 40 degrees to reality.

    For an elderly AC type, 25mA is in spec and not bad going.

    Then there is the odd mA of measurement rounding or meter uncertainty that I am not bothering with.

    hope this sets your mind at ease, there is almost certainly nothing bad going on,,

    Mike

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    It is not impossible, lets assume that most of the 15mA of standing leakage is capacitance L-E due to capacitors within mains filters in electronic loads plus  the natural capacitance of the wiring, which may also be on the high side, given it is a whole site figure.

    This current is at right angles to the applied voltage, or if you prefer, offset in time by a phase shift of 90 degrees.

    The insulation test tells us that less than 1mA is actually resistive leakage (0,25M ohms would do that.) so lets assume it is to all intents and purposes all capacitance.

    The ramp test however will be dropping in a binary weighted set of resistors or similar so the current it induces L-E will be in phase with the incoming voltage, and that stopped at just over 20mA.

    Adding the two, correctly, with a bit of pythagoras, we get Sqrt (20*20 +15*15 ) = sqrt (400+ 225)= sqrt (625) = 25mA at about 40 degrees to reality.

    For an elderly AC type, 25mA is in spec and not bad going.

    Then there is the odd mA of measurement rounding or meter uncertainty that I am not bothering with.

    hope this sets your mind at ease, there is almost certainly nothing bad going on,,

    Mike

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