An interesting read on RCD’s

[Dozer 73]

Sent this today by our engineering manager , an interesting read regarding rcd protection
RCDs: everything an electrician should know - IET Electrical - http://electrical.------.org/wiring-matters/68/rcds/index.cfm

 

[Deleted member 26818]

Type B RCDs are still pretty rare, but not as rare as type B RCBOs.

 

[haptism]

Good article; then if a circuit has USB sockets, then what type RCD would it need. Thought it would not need other than usual AC type as the USB ELV bit is separated from LV (at least the bonafide ones) are they not. ?

 

[gazdkw82]

Great article.

I will defo brush up on the different types of RCD’s. It’s not very well known about the different types. I would never have known the wrong rcd in the wrong installation could render it useless

 

[suffolkspark]

so is it saying if an installation is fitted with the most basic types of rccb or rcbos, then later say solar pv is installed,,,, then they all need changing because they wont work properly?

 

[Deleted member 26818]

so is it saying if an installation is fitted with the most basic types of rccb or rcbos, then later say solar pv is installed,,,, then they all need changing because they wont work properly?

Yes.

 

[lurch]

very interesting.......... very surprised my inspector has never brought this topic up. I feel a non conformancy coming on . . . .

 

[lurch]

. . . . off topic here, but cant find the "post a new topic button". . . . help!

 

[Richard Burns]

. . . . off topic here, but cant find the "post a new topic button". . . . help!

Post new thread still seems to be in the top right hand corner of the page, has yours disappeared?

 

[Deleted member 26818]

Nothing there for me.
Found it.
Have to click on home, then forums, then the forum you want to post in.

 

[Ian1981]

so is it saying if an installation is fitted with the most basic types of rccb or rcbos, then later say solar pv is installed,,,, then they all need changing because they wont work properly?

Where a microgenerator having a d.c. source does not incorporate the equivalent of a transformer providing at least simple separaton between the d.c. and a.c. sides, an RCD installed for fault protecton by automatc disconnecton of supply or for additonal protecton (IΔn ≤ 30 mA) must be of a type that will operate as intended in the presence of d.c. components in the residual current. (This does not apply where it has been established – such as from a specifc writen statement given by the inverter manufacturer – that the inverter provides galvanic isolaton between the d.c. and a.c. sides that prevents it from feeding d.c. current into the electrical installaton.)

 

[gazdkw82]

So an rcd in this type of installation...would it still pass trip tests? Ramp tests?

 

[suffolkspark]

so are PV installers trained so as to be aware that they're installations could render an existing electrical one unsafe? and also on larger installations where a solar PV set may be running off 1 did board, is every other rcd/rcbo in the installation effected of will the DC current just bugger off upstream? where is it trying to go.... ?

 

[lurch]

Post new thread still seems to be in the top right hand corner of the page, has yours disappeared?

thx. . . .

 

[Deleted account]

This is exactly why I am part of this brill forum.

Nice one dozer73

 

[Leesparkykent]

A PV circuit shouldn't share the same RCD as other final circuits anyway. As the required disconnection times will probably not be achieved.

 

[suffolkspark]

any more expertise on this chaps and chapettes? ive actually looked through a few catalogues and they don't even list what type of rcd they are in most of them!

A PV circuit shouldn't share the same RCD as other final circuits anyway. As the required disconnection times will probably not be achieved.

so its just the one on that circuit that needs to be the b+? rest of installation is normal?

I've spoken to 5 other electricians this week, nobody had come across or been taught this either!

 

[Deleted member 26818]

No trip tests are conducted by saturating the trip coil on A and A.C. type RCD/RCBOs with d.c. current.
The No trip test (as far as I am aware) will not function on B type RCD/RCBOs.
With p.v. installations, the Inverter manual should tell you which type of RCD/RCBOs are required.
If the manual states B type RCD/RCBOs are required, then it will apply to all RCD/RCBOs used in the Installation.
A and A.C. type RCD/RCBOs will still operate, as long as there is no d.c. current from the p.v. saturating the trip coil.

 

[Leesparkykent]

any more expertise on this chaps and chapettes? ive actually looked through a few catalogues and they don't even list what type of rcd they are in most of them!



so its just the one on that circuit that needs to be the b+? rest of installation is normal?

I've spoken to 5 other electricians this week, nobody had come across or been taught this either!

The inverter can take up to 5seconds to disconnect so if the circuits on the same RCD require a maximum of 0.4 seconds disconnection then you can't guarantee this will be achieved. For the use of type B RCD's with PV then refer to reg 712.411.3.2.1.1 same With vehicle charging points may require a type B RCD 722.531.2.101.

 

[Lucien Nunes]

The references to USB sockets and most types of SMPSUs associated with networking etc seems to be a red herring, as they must provide electrical separation between the AC input and DC output. Therefore, they are no more likely to inject a DC current into the circuit conductors that would saturate and disable a type AC RCD, than any other electronic load. On an SELV supply, there would have to be an input-output fault (nasty, possibly live iPhone!) as well as a fault to earth from the other side of the DC supply, and the unit would still have to carry on functioning to produce the offending DC. Possible, but in practice the whole thing would likely blow to smithereens and no longer generate DC. The input rectifier of an SMPSU can produce a DC load if one side of the non-separated DC bus leaks to earth and one leg of the bridge blows out (or it's half-wave), but this should not result in shock because the electrical separation remains, so it doesn't rely on the RCD for protection. It could disable the RCD from protecting against other faults, but again the 99% likely result is that the bridge will simply short out completely and blow the internal fuse / circuit OCPD, leaving the RCD to function normally.

Phase-angle controlled loads are more likely to have a DC component in their supply current and there were, maybe still are, appliances that deliberately produce one. For example, hairdryers with two heat settings that use a diode to halve the element mean current, draw their entire load as DC on low heat. Does this prevent operation of a traditional type AC RCD? I would think it probably does, without there even being a fault. I would actually like to try this (anyone care to do it?) Put a half-wave low heat thingy on a type AC and run a normal RCD test?

I'm familiar with RCD selection on the basis of DC and HF load current components from theatrical and machine installations. Large lighting dimmers can produce a significant DC component in their supply current, especially in the event of an output device fault leading to half-wave loads of many amps. For these, a type A may provide adequate protection, but for a device that can fault from the DC buss of a non-separated bridge rectified mains supply, e.g. an inverter, the smooth-DC sensitivity of a type B is needed.

In summary, yes, food for thought if you're not familiar with DC-sensitivity of RCDs, but I am not fully in agreement with some of the comments.