# RCD tripping - Have i found the cause?

In a perfect installtion the NE current from an NE would flow back proportionally to N and to E in a TNC-S system or indeed any TNS system containing a NE path for such a circulating current that makes it act proprtionally TNC-S like in principle.
In practice this is less likely to cause sufficient current missmatch of N to cause RCD trips or greatly slanted tripping characteristics.

So it is usually L & N imbalance caused by any circuit on the same RCD and therefore any L to fault or N to E fault on a RCD circuit drawing current to achieve the imbalance for a trip (say 30mA notationally, being anything over 15mA up to 30mA usually and often set as tripping between 21mA to 24mA or from 24mA to 27mA according to the ramp tests on my meter - on 30mA setting it starts at 15mA and ramps up 3mA per step until it trips) so if we reckon most RCDs might be considere 21mA to 27mA sensitive in practice on a bog standard 30mA non delayed RCD.

Well I think, unless anyone else has some thoughts.

The neutral and earth with no load are the same voltage, however as you load the circuit, the neutral voltage raises towards the line voltage due to resistance (impedance) in the cable. So with a neutral to earth fault the more current on the circuit the more current will flow neutral to earth. So if the fault is in a low load appliance it may not trip the RCD until a high load appliance is used.

There are three test instruments, the RCD tester tests the RCD, we will assume the RCD is OK, so we have the insulation tester and the clamp on ammeter the insulation tester used 500 volt DC the problem is it uses DC so will not pick up leakage due to capacitive or inductive leakage, and the clamp on will only measure if the RCD has not tripped, so both have their place. The problem is the yellow tester to left (my old one) measures is 0.01 increments, this was good enough when I bought it working in Hong Kong before take over, but today we need to measure in 0.001 increments to show what the leakage is in mA. I had to hunt hard even today to find one that measures in 0.001 amp increments, I also wanted DC so likely paid more than needed, cost me £35 the UT210 comes in a few versions, I have the UT210E which will measure DC there is a better one which also measures frequency, and some cheaper ones which don't measure DC.

But without some test equipment there is a lot of guess work, the insulation tester also cost me £35, you don't really need both, but you do need some thing to narrow down what is faulty, to hire will likely cost more than to buy, the reason is to hire they need a calibration certificate, and it is the testing after each hire that costs, since your not filling in a test certificate you don't need it calibrating.

The question is which is cheaper, buy test equipment which you may never use again, or get an electrician who has the test equipment? I love my clamp on, it is so easy to see how close to the wind one is sailing, we for years used RCD's and MCB's and it may just be the small amount of leakage on each circuit is just adding up so too close to the limit, my own house total shows around 24 mA leakage, so with a single RCD it would not take much more to trip them, they will normally trip between 24 - 28 mA but my house split between 14 RCBO's (MCB and RCD combined) so each RCBO is well within the 9 mA limit suggested as the limit for a 30 mA circuit.

We would expect well over the 1MΩ minium for each circuit, more likely 10 - 100MΩ, but capacitive and inductive leaking is not tested with DC, so with an AC supply there is always some leakage. I would find in old house I would get a batch of trips, then not trip for years, never did find why, but did not have the red and black clamp on then. So this house one of the first things was swap to all RCBO, OK it was expensive, but likely only the same as loosing my freezers when a RCD trips, living 8 miles from main shops I keep a lot in stock, so would think around a £600 bill to re-stock. Not sure if insurance would cover that bill?

The neutral and earth with no load are the same voltage,
Not always true and the possible exceptions need to be taken into account when searching for the reason an RCD is tripping.

First thing to consider is where the "Earth" wire, the CPC, is connected to,
(1) In a PME installation the "Earth" is connected to the Neutral where the supply cable enters the company fuse. This "Earth" may not be the same voltage as the True Earth which is the Ground the house is built on. In fact it is very seldom is the same voltage due to the current flow along the network Neutral to all the houses on that street cable.

(2) In a TT installation the "Earth" is connected to the Ground using a Ground electrode ( Earth Rod )

Second thing to consider is whether the leakage current is flowing to "Earth" ( the CPC ) or to True Earth ( Ground )

In a perfect installtion the NE current from an NE would flow back proportionally to N and to E in a TNC-S system or indeed any TNS system containing a NE path for such a circulating current that makes it act proprtionally TNC-S like in principle. .... In practice this is less likely to cause sufficient current missmatch of N to cause RCD trips or greatly slanted tripping characteristics.
True -although, as below, I don't think it is very much "less likely"
So it is usually L & N imbalance caused by any circuit on the same RCD and therefore any L to fault or N to E fault on a RCD circuit drawing current to achieve the imbalance for a trip (say 30mA notationally, being anything over 15mA up to 30mA usually and often set as tripping between 21mA to 24mA or from 24mA to 27mA according to the ramp tests on my meter - on 30mA setting it starts at 15mA and ramps up 3mA per step until it trips) so if we reckon most RCDs might be considere 21mA to 27mA sensitive in practice on a bog standard 30mA non delayed RCD.
Considering the simplest case of TN-C-S (and a neutral-CPC fault) , the only difference between the "intended neutral path" (from a load protected by a different RCD) and the path via the N-E fault is the "R1+R2" (from the fault) in the circuit with a fault - so I would expect 'a (perhaps very) significant proportion' of the current to go through the fault path. ... and not all that much different with TN-S

It obviously depends upon the current being drawn by the load - but if it were,say, about 1 kW (about 4.3 A), then it would only seed about 0.7 % of the neutral current to flow through the fault to create a 30mA imbalance in the RCD (hence only about 0.35 % for a 15 mA imbalance, which could be enough to trip the RCD).

I therefore don't think that such a trip would be particularly 'unlikely'.

Kind Regards, John

Not always true and the possible exceptions need to be taken into account when searching for the reason an RCD is tripping.

First thing to consider is where the "Earth" wire, the CPC, is connected to,
(1) In a PME installation the "Earth" is connected to the Neutral where the supply cable enters the company fuse. This "Earth" may not be the same voltage as the True Earth which is the Ground the house is built on. In fact it is very seldom is the same voltage due to the current flow along the network Neutral to all the houses on that street cable.

(2) In a TT installation the "Earth" is connected to the Ground using a Ground electrode ( Earth Rod )

Second thing to consider is whether the leakage current is flowing to "Earth" ( the CPC ) or to True Earth ( Ground )
I must of course agree, however my point is because the RCD trips when a kettle is used, does not always mean the kettle is at fault, it could be some damp toast left in the toaster which is left plugged in, as the built in switch on toaster only switches the line.

It however may be the kettle at fault, but without test gear, one simply does not know, hitting a nail with a shoe heal may drive in the nail, but can also damage the shoe and the nail, so better to use a hammer. So the same applies to electrics, some times trial and error may work to find the fault, but better using test gear.

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