LAP Voltage Tester RCD Test function

[CAWORK]

Good Evening, I have purchased the two pole version from Screwfix, it seems good enough for a voltage tester which is all I needed it for, but would like to know how the RCD Test works, is it via a high value resistor like an RCD Test button between L&N, therefore only checking the mechanical moving parts, or does it inject a current of 30mA into the earth circuit? (Like a lot of advanced socket testers do, the instructions state to probe between hot and ground..).

Regards

 

[flameport]

It will just be a resistor which causes more than 30mA to flow when the button is pressed. Same as pressing the test button on the RCD itself. You will need to connect the probes to L and E for it to work.

 

[CAWORK]

It will just be a resistor which causes more than 30mA to flow when the button is pressed. Same as pressing the test button on the RCD itself. You will need to connect the probes to L and E for it to work.

Many Thanks

I didn't expect this product to be very sophisticated, regarding its price, however the instructions ('Americanised' state to probe between Hot and Ground, so L & E, which is what I've done).

Saying that though, isn't an actual RCD Test button resistor linked between L & N, so creating an imbalance, although no actual leakage to Earth? (Regarding this, how does an Advanced socket tester inject an Earth leakage current of approximately 30mA into the Earth circuit, thinking about it, is this done via a resistor?).

Regards

 

[plugwash]

Saying that though, isn't an actual RCD Test button resistor linked between L & N, so creating an imbalance, although no actual leakage to Earth?

Yeah in most cases it is (i've heard there are some RCD sockets which do send the test current to earth).

The key is it's neutral upstream of the RCD and live downstream (or vice-versa). So the test current only flows though the rcd coil in one direction.

You can do the same with an external tester as long as you have access to appropriate probing points both upstream and downstream of the RCD. Just use "upstream neutral" instead of earth,

 

[ericmark]

I do see the problem the instructions show with a German "Schuko" socket that could not possibly test the RCD. This is what I would expect to see.
I found a second Set of instructions which seem better. However the specifications

say operation time 30s maximum, this does seem odd, most test probes you can use for as long as one wants.

I remember a metal clad MK socket with built in 10 mA RCD where pressing the test button would take out the 100 mA RCD in the main board, at 207 - 253 volt the resistor would need to be 6.9 kΩ which would give a load between 30 and 37 mA depending on voltage, likely it would be slightly lower than that, but over the 4.1 kΩ which would be the no trip value for a 100 mA RCD, at 2 kΩ it would trip a 100 mA RCD at 207 volt which is seen as minimum UK voltage.

However since it does not actually say what load is imposed, it seems rather pointless. Which is what I thought about the test button on the MK socket, since it was an active socket, when the 100 mA RCD tripped it would trip anyway, so rather pointless having a test button.

 

[winston1]

Saying that though, isn't an actual RCD Test button resistor linked between L & N, so creating an imbalance, although no actual leakage to Earth?

No, absolutely not. That will not create an imbalance. Think! The load is connected between L and N.

 

[ericmark]

No, absolutely not. That will not create an imbalance. Think! The load is connected between L and N.

Sorry yes it does

number 4 being the test button. With most RCD's there is no earth connection.

 

[winston1]

Your connection shows line output to neutral input which is not the same as live to neutral (both outputs).

 

[bernardgreen]

The key is it's neutral upstream of the RCD and live downstream (or vice-versa).

It is important that the test resistor is connected down stream from the RCD disconnector as in this sketch

IN

OUT

While the test button is pressed and the RCD has not yet operated the test resistor is dissipating about 7 Watts. This amount of heat can be absorbed by the thermal mass of the resistor provided it is only a short time ( less than a second )

If the arrangement is

OUT

IN

then the resistor will have mains applied to it for as long as the button is held pressed. Very likely to become dangerously hot before burning out.

Some RCDs have a third contactor which ensures the resistor is disconnected when the device trips irrespective of the way the RCD is connected

 

[JohnW2]

It is important that the test resistor is connected down stream from the RCD disconnector as in this sketch .... While the test button is pressed and the RCD has not yet operated the test resistor is dissipating about 7 Watts. This amount of heat can be absorbed by the thermal mass of the resistor provided it is only a short time ( less than a second ) ..... If the arrangement is ... then the resistor will have mains applied to it for as long as the button is held pressed. Very likely to become dangerously hot before burning out.

If they are wired per your diagrams then, yes, that is a potential issue - but are they usually wired like that? On the face of it, unless I'm missing something, the potential problem could be removed by connecting the resistor to the 'other side' of the "L switch" of the disconnector, couldn't it?

A problem which I think is unavoidable (regardless of 'wiring') arises if the RCD fails to 'trip' as a result of pressing then test button. In that situation current will continue to flow through the resistor for as long as the test button remains pressed.

Kind Regards, John

 

[ericmark]

With a commercial RCD I have seen where the current transformer is not built into the device opening the circuit, even with a remote test button, and options to set tripping current and time delay, also physically connected to a MCB so works like a RCBO.

However with domestic it is not permitted to give the user the option of setting tripping levels, so we only use all in one units.

The problem is a passive RCD needs voltage to work, if a earth fault also causes a volt drop, then the RCD may fail to work, in theory should work at 50 volt, and below 50 volt safe enough anyway, but I tried to use RCD's with 55-0-55 volt supplies and found they failed to work, and had to get special 110 volt versions, even they failed to work when one of the lines had failed, so in real terms only option was an active RCD so loss of voltage it trips anyway.

Hence most plug in RCD's were the active type. I fitted many active RCD's not because I wanted RCD protection but wanted no volt release for things like bench grinders.

However as far as this tester goes, 30 seconds use then 10 minutes before it can be used again seem crazy, remember it does not say 30 seconds use of RCD test button, it simply says 30 seconds use, who in their right mind is going to want to wait 10 minutes between each test for voltage?

And what is the point of a RCD test button when it does not tell you what the test is?

 

[bernardgreen]

Eric

A passive RCD does not need voltage to operate, it requires alternating CURRENT through the sensor coil. That current creates an alternating magnetic flux in the magnetic core which in turn creates voltage in a winding on the core. This voltage energises the electromagnet that trips the mechanical switch that disconnects the supply.

Yes being pedantic the current flowing through the impedance of the sensor will create a voltage.

The requirement for a delay after operating the test button is to allow the resistor in the test circuit to cool down.

I wanted RCD protection but wanted no volt release

Then I would suggest a separate contactor and reset button to provide the No Volt Release

 

[ericmark]

I also thought the RCD worked on current, maybe some do, but to get enough energy from around 26 mA to open a latched set of contracts seems to be stretching it a bit, at 50 volt that is 1.3 watt so not impossible, but in the early days I did try using a standard RCD on a 110 volt supply and they did not trip with a test resistor.

In theory it is only the maximum voltage that matters, over 50 volt AC is low voltage so if the supply is over 50 volt it should work, same applies when proving dead, proving unit must test at 50 volt to show the tester can detect 50 volt, having 250 volt as well may show all lights work, but the important thing is tester shows if over 50 volt.

Since our standard 110 volt three phase supply is 64 volts to earth, and where I was working when doing tests our 110 volt was three phase, the RCD should have worked. However the RCD tester was designed for 230 volt, so could not use the standard RCD tester as it simply refused to work on 64 volt. I used a 1kΩ line 1 to earth which at 50 volt is 50 mA so should have tripped, but did not.

Today we can buy 110 volt RCD's but pre internet days, getting a 110 volt version was like getting that stuff which rocking horses don't make. But if a standard 230 volt RCD will work down to 50 volt, why make special 110 volt versions? Looked at data sheet seems to be nothing as to why need a special 110 volt version.

I will guess there are some electronics in the RCD which do need voltage to work,

Wikipedia the sense circuitry 7 seems to have some electronics, I would assume these need a voltage to work.

 

[bernardgreen]

The sensing core ( 6 ) in the image has 1/2 turns on Live and Neutral which is unlikely to create enough power in the secondary to operate ( trip ) the mechanism. Hence some amplification is needed.

If there are several turns on Live and Neutral through the sensing core with a secondary coil of several tens of turns then there will be adequate power to trip the mechanism.

 

[JohnW2]

Eric ... A passive RCD does not need voltage to operate, it requires alternating CURRENT through the sensor coil. That current creates an alternating magnetic flux in the magnetic core which in turn ....

I think that, despite his choice of words, eric probably did not literally mean "voltage" (as opposed to current) but, rather, just 'electricity'.

I suspect the point he was trying to make was that most/all RCDs/RCBOs these days have electronics within them (rather than have the sensor coil current directly operating the 'disconnector') such that, in the absence of 'electricity' (i.e. an L-N potential difference), the device would not work.

This was, as I understand it, why some RCBOs (but never any 'RCD' that I've seen) had a 'functional earth', so that in the event of 'no electricity' (as with a 'lost TN-C-S PEN' such as you so frequently mention) the electronics of the RCBO could remain operational, powered by the L-E PD, even if the neutral were 'broken' or not connected. However, it seems that many/most manufacturers have now decided that the probability of a 'lost PEN' and an L-E fault within an installation happening simultaneously is so vanishingly small that a 'functional earth' for this reason is not really necessary/justified.

In passing, you don't seem to have responded to my comment/question about the 'wiring' of RCD test buttons.

Kind Regards, John