:-)
Oh sure, in normal operation the current imbalance can arise because some of the current returns via the earth conductor, or the actual muddy earth, or the "wrong" neutral. I was just talking about how the test button would cause an imbalance (without requiring an earth connection to the RCD itself).
I am not sure the suggestion (in itself) holds much merit. In many cases you would be fusing the plug at or near that of the circuit. The result would be an inability to discriminate in the event of a fault. This would just increase service and rectification times.
It would only be a worthwhile exercise if you also increased the current capacity of the standard circuits. However that would at a stroke require all the plugs used on it to be fused. So it seems unlikely to happen since the changeover would be too costly etc.
Yup, the original reason for the use of ring circuits was partly to save copper. It was also IIUC, the intention to be able to supply enough power for day to day needs and also heat a house if required. Hence >=
7kW per circuit, and circuits being able to cover a wide floor area.We don't here either - with rings or radials.
The circuit designs assume diversity - i.e. the assumption that not every outlet will require the full 13A. This matches real world usage rather well.
I do, because a ring is not the only circuit that requires protection at the plug. Also there is nothing intrinsic to the ring itself that requires the fusing, only the magnitude of the current. Since radials of equal power are also permitted in our wiring regs, it seems inappropriate to attribute the requirement for fusing to a ring circuit.
You can't just consider plugs in isolation. The 13A plug was one part of a whole new wiring scheme, which included the circuit layout, circuit protection, earthing, area power requirements, etc.
Indeed, so a non starter really...
Correct.
They might but then they are not producing a reliable test of the current transformer. The above test relies on an external circuit (the Earth) that an RCD does not require.
True enough but read what I wrote not what you think I wrote.
I'd rather not, your basic understanding seems to be some what lacking.
Heh. So if you need a radiotherapy machine sorting don't come to me then...
-- Malc
AIUI you either need a resistor to earth, which could potentially raise the earth voltage to mains if its faulty, so would be unwise, or put a resistor across one side of the current transformer, diverting some of the current and tripping the device. Maybe it would be easier to just short out a few turns to make it unbalanced?
Neither putting a resistor across one side of the current transformer nor shorting part of the current transformer would allow the trip test button to work without an operating load. Why not just listen to Dave?
Tony
No wonder the health service is in a state, then.
You stated you need to provide an earth path to test an RCD in response to Mr Liquorice's post. You patently have no knowledge of a domestic CU otherwise you'd know there is no earth connection to the RCD(s) - yet a test button is provided.
Maybe, but I didn't say it didn't put a load across the coils too. I pointed out how to get an imbalance not how to make one.
Thanks, that all makes perfect sense to me. Is that 2 or 3 metre flex length limit for a 16A breaker or for a 32A breaker? And with such a short flex would it be permissible to use a 13A fuse in the plug?
The trouble with this thread is that you're both right!
but
I hope that this clears up the working of the RCD which will therefore work whichever polarity is connected to it. So now we're back to the original problem of getting the polarity correct when connecting a continental supply to a UK & Ireland caravan.
Pedro
It's for a 16A breaker. Any modern appliance with a 13A plug also needs to remain safe when moved elsewhere in the EU and connected to a 16A outlet without a fused plug.
You are correct that in theory you can use a 13A fuse in all appliance plugs nowadays, except for old appliances with thin flexs which are longer than EU regs now allow, and extension cords.
TBH the thought of someone with your lack of basic knowledge, or perhaps the inabilty to admit a mistake, being allowed to mess with radio therapy machines worries me.
Which brings us back to RCDs not being polarity sensitive and if tripping when connected with "reverse" polarity indicates something a miss with the caravans wiring after the RCD.
I suspect that the incoming "neutral" is bonded to the caravan chassis and the chassis is also connected to the incoming "earth". This is incorrect, after the RCD the three supply wires "live", "neutral" and "earth" should be kept and remain seperate. If a given the supply only provides "live" and "neutral" then the caravan chassis must be connected to real ground with an earth spike.
That's not what Dave wrote.
A resistor between L&N, but bypassing half of the current transformer's bifilar winding (this is what Dave's diagram showed), _will_ trigger the RCD without any load connected.
Shorting half of the transformer will need a load current too, agreed.
Maybe you are not using a threaded reader - I was replying to Dennis and contradicting him and I was agreeing with Dave.
Tony
The UK & Ireland Wiring Regulations specifically forbid any connection other than the PE (earth) to be made to any exposed metalwork in/on the caravan. This means that it is illegal to connect the neutral to the chassis either before or after the RCD. In the event of a polarity reversal someone could be killed just stepping into or out of the van.
A caravan without an RCD (it is not legal to sell a van without an RCD in the UK & Ireland and I think in western Europe) 'SHALL have' a protective earthing spike which has been tested to ensure its effectiveness in meeting the requirements of the Regulations. Clearly this is not practical with Joe Public pitching his/her tourer. I can just see the site owner's face when asked "What is the prospective short circuit capability of your supply and what type and rating of protective device is immediately upstream of your point of user supply?" On some sites I've been on this would then result in me driving in at least 3 metres of earth rod alongside the van - probably going through a cable or pipe at that depth!!
Before someone queries the first sentence, it is legal for the 12 volt system to have one pole (usually the negative) connected to the earth/chassis. Some manufacturers do this others don't. It can slightly increase safety under certain fault conditions.
Pedro
Yes he was contradicting something that was correct. Best leave him alone and see if he learns anything. 8-)
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