Sparks wrote:

That's a bit ambiguous - do you mean the current that flows through your
meter with the meter leads connected between the neutral and earth
terminals, with the earth connected? (Or if not, then what exactly?) A
neutral-earth s/c current measurement like that isn't very helpful - what
you need to do is to measure the current in the earth wire of the
appliance. IOW connect the earth wire from the hob to the earth terminal
of the cooker connection unit via your meter.


Yep, this is exactly what I did Disconnected the earth from the connection
plate, then clipped one test lead to it, and the other the Neutral in the
connection plate (as connecting it to the earth in the plate trips the RCD)
I also tried clipping the test lead to the metal body of the hob and the
neutral in the connection plate - both displayed the same results of 2.9A
flowing (The test lead produces small sparks when connected too)

Whoa - not the way to go. Remember that when you have your meter set to
read current it looks like a short circuit across its probes (because it
is meant to be placed in series with a circuit - not across it like when
measuring voltage). If I understand what you have written correctly then
you have just created an earth neutral short. A sure recipe for tripping
a RCD in many cases. What sort of earthing do you have?

What you need to do is leave the live connected to the live, neutral to
the neutral, and then disconnect the earth. Now connect one test lead to
the earth connector on the appliance, and the other one to the earth
wire you just disconnected. That will let you measure the current
flowing to earth. You will probably need you meter on a lower range! You
may still get a trip when you do this - but you should be able to get a
reading for the leakage current. You could try turning off other RCD
protected circuits at the CU for the duration of the test to desensitise
the RCD (by removing other sources of imbalance/leakage) if required.

Sparks (who wants to RCD everything possible, now more than ever!)

Why?


Because if the hob is faulty, as I expect it is, this fault would not have
been identified as the leakage would not trip the 32A MCB - Say at some
point the earth gets disconnected for whatever reason - the chassis of this
hob will be ready to supply a user with 2.9A @ 120v - and as it is only
leaking 2.9A no protection device would trip.

If it is faulty, then passing 2.9A to earth would be a rather odd fault.
I expect you are simply measuring a large current flow between N and E
as a result of a slight difference between your earth and neutral
potentials. You would also not have been able to use the cooker to boil
your pan of water unless your RCD was also seriously knackered as well
since 2.9A should trip a 30mA threshold device rather rapidly.

(I take it it trips when you hit the test button?)

The 120V your read sounds like capacitive coupling. It is hard to think
of a fault in a typical suppression circuit on the input of an appliance
like this that would give you a 2.9A (i.e an 90 ish ohm resistance from
live to earth) that would not result in something in the filter going
bang in short order, or preventing the cooker from working.

Surely it has to be safer to provide more protection than is actually
needed?
(If not, please tell me why!!)

Several reasons - you have already found one of them. Heating devices
like this are often have high leakage currents (and we are talking
milliamps here - not amps), which means you are eating into the leakage
allowed by the RCD for all its protected circuits. The net result a
greater likelihood of nuisance trips. RCDs are designed to protect you
from electrocution in situations where there is a significant risk of
you making contact with live - i.e. portable appliances, with the worst
culprits being power tools used outside. For a permanently wired fixed
appliance this will be very unlikely and the risk is minimal.
Overcurrent protection and decent earthing gove you more than adequate
protection (unless you are on a TT installation).


--
Cheers,

John.

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