Is this article a Red Haddock ???

[flameport]

Why should that trip a type ac RCD or any RCD?

It won't trip an RCD, but could prevent an RCD from tripping when a fault occurs.

 

[JohnW2]

Why should that trip a type ac RCD or any RCD?

It wouldn't - but see flameport's reply (but also my reply to him which I am about to write).

 

[JohnW2]

It won't trip an RCD, but could prevent an RCD from tripping when a fault occurs.

As you will be aware, I've been trying hard to gain some understanding of the various 'Types' of RCDs, but am coming to wonder whether Type A ones are actually 'good enough' to achieve very much in many situations - so perhaps you can help me to understand? ...

BS7671 says:
.... For RCD Type A, tripping is achieved for residual pulsating direct currents superimposed on a smooth direct current up to 6 mA.

If that is true then the implication is that the RCD might well not trip if the underlying DC component of current is greater than 6mA. In the context of what we're discussing (a motor fed through a diode) I would imagine that the DC component is likely to be a lot more than 6mA, so does that mean that a Type A RCD would offer little/no benefit (over a Type AC) in such a situation?

 

[Harry Bloomfield]

Why should that trip a type ac RCD or any RCD?

It will not cause it to trip, rather it will slightly affect its ability to trip.

 

[JohnW2]

It will not cause it to trip, rather it will slightly affect its ability to trip.

As I've just written, on the basis of what little I know of the characteristics of a Type A RCD, it might even seriously affect the ability of a Type A RCD to trip!

 

[SUNRAY]

Oh wow, it's strange to see someone else asking the same question I asked 100 years ago

 

[JohnW2]

Oh wow, it's strange to see someone else asking the same question I asked 100 years ago

I've been asking questions about the behaviours of these different Types of RCD ever since I first heard of them and, despite having done a fair bit of reading, I'm still not very much the wiser.

On the basis of what little BS7671 says, Type A ones are only guaranteed/'required' to respond to a residual current in the presence of a DC component of up to 6mA - which is presumably relative peanuts in the sort of context we've been discussing here. On the other hand, BS7671 tells us nothing at all about the performance of Type AC RCDs in the presence of DC components (of any magnitude0 - but the implication appears to be that DC components even less than 6mA may impair their functioning - which, if true, would rather surprise me.

 

[fixitflav]

For modern electronics which use DC, there is a small loss converting AC to DC. However this is tiny, and even if properties were connected to DC supplies the conversion would still be required, as whatever voltage was supplied to a building would not be what was required by the electronic device.

Not only that, but the energy used by electronic devices is likely to be a small fraction of the total.

 

[ericmark]

Someone sent me 531.3.3 from BS7671:2018 where it goes through types AC, A, F&C with 0 mA, 6 mA, and 10 mA and I have tried to measure the DC leakage

with the meter shown, with AC not a problem as seen here, but with DC the meter needs to be zeroed, and any slight movement, and it needs zeroing again, so I have failed to measure DC leakage, would be interested to hear how others have got on measuring DC superimposed on the AC supply.

Even 10 mA is not much, and it would be easy for this to be exceeded, and the only other test one could do, is to test the RCD with all loads connected, and confirm if the RCD works.

I know with an EV charging unit we need a RDC-DD, but the question is what about the rest? Well I suppose a RCD is secondary protection with a TN supply, only with items outside and with a TT supply, is the RCD primary protection.

 

[flameport]

whether Type A ones are actually 'good enough' to achieve very much in many situations

They are better than Type AC, but only of use in situations that do not have smooth DC.
For most domestic properties they are good enough today, and certainly far better than the AC types of the past.
They will not be good enough in the future as more electronics, inverter drives and DC items are installed.
Eventually all RCDs will be Type B+ or some equivalent.

BS7671 tells us nothing at all about the performance of Type AC RCDs in the presence of DC components (of any magnitude

It won't, and neither will anything else as they were never designed for or tested with anything other than 50Hz sinusoidal AC.
Their behaviour with anything other than 50Hz AC is undefined and unknown.
Given the options of testing all makes and models of AC RCDs with various other currents or just banning the whole lot, most countries went with the ban several decades ago.

Type A ones are only guaranteed/'required' to respond to a residual current in the presence of a DC component of up to 6mA - which is presumably relative peanuts in the sort of context we've been discussing here.

Type A will work with pulsed DC which is what you get from a diode, there is no 6mA limit there.
The 6mA refers to where a continuous DC current exists, such as that you would get from a battery or solar panel, which is why installations that have those will have other types of RCD designed for DC.

A fixed DC current will saturate the magnetic core in the RCD and over a certain level it will not work, even if the fault current is AC. As most modern RCDs have tiny cores, it only takes a current of a few mA or 10s of mA to saturate the core and completely disable the RCD.
This does not happen with pulsed current as the magnetic field is not constant.