RCDs/RCBOs in series

Becuz some numbnuz would go ya init uze da chillz sockz, jus plgz da power drilz in ere for putzin up da shelf, den plugz da chill back in. init

 

If a customer refuses to let you connect a dedicated freezer circuit, their circuit, to their RCD then you can't force them to. Its not dangerous, its a deviation.

The freezer socket has to be I accessible to casual users. It would be behind the freezer out of the way. There's no point keep debating it, its allowed if its necessary.

 

Oh hey i not disagreeing with you. Yes customer is as always right.
You just have to do the install with as close to the guidelines as possible for a non RCD protected circuit. For instance run it in within steel conduit in or on the wall and use metal clad back-boxing etc. Usually the extra price tends to put them off as cladding and conduit wire don't come cheap.
Even if you use SWA cable with glands into metal clad boxes, it is still a pricey job to feed one single freezer circuit but hey, how deep is your pockets.

Just make sure it is clearly labelled as it covers your ***, oh and take pictures of it. People remove labels and say you never installed a label.


Specifically labelled or otherwise suitably identified socket-outlet

The exception in indent (b), relating to the omission of RCD protection for specific socket-outlets, is included in both the current and new versions of the regulation.
It is stressed that the exception may only be applied for a socket-outlet that has been provided for the connection of a particular item of equipment. The socket-outlet must be labelled or otherwise suitably identified so that users will be clearly informed that the socket-outlet is intended only for plugging in that particular item of equipment and for no other purpose. Unless the electrical installation designer is convinced that the socket-outlet cannot reasonably be expected to be used for other purposes, RCD protection for that socket-outlet should not be omitted.
In the vast majority of cases, it should not be necessary to omit RCD protection for a socket-outlet. For compliance with Regulation 531.2.4, a socket-outlet should be connected to an RCD that serves a sufficiently small number of other socket-outlets or items of equipment, so that any protective conductor current that may be expected in normal service will be unlikely to cause unwanted tripping of the RCD.

 

Deviations can only be used if the deviation affords the same level of safety as BS7671. You cannot just deviate from BS7671 and put down 'the customer requested it'

 

I think some people need reminding that the OSG is not (nor has it ever been) the Wiring Regulations.

It is simply a guide book with pictures in it.

 

I have to disagree here. Two BS EN 60898 circuit-breakers (still commonly referred to as "MCBs") in series will not discriminate under fault conditions.

 

Say you have a 40 amp mcb as the front, and then next to it you have a 6 amp (in series) and you throw a fault at them ie l to n on outgoing - then its pot luck which will throw first?

 

If a customer won't let you do work, it has to be logged somewhere that it doesn't comply.you cant force a customer to have trunking up their wall and anyway I don't get this needing an rcd for a fridge socket just because it's less than 50mm deep. It's nonsense. Where's that in the regs

 

You mean the cable be buried in the wall less than 50mm deep without mechanical protection would under new regs require it to be RCD protected

It does under these conditions need be RCD protected.
If you run steel conduit into metal clad then RCD may be omitted or the use of SWA cable into metal clad back boxes and have the correct warning labels. This would not require RCD protection, reason? Encase someone drills into a wall to fit a shelf and drills into the cable.

Check the amendment 3. I am sure i posted the link somewhere.

 

I was talking about in that post an MCB being in the dwelling property connected to a stretch of SWA cable going into a garage mini consumer unit with a main switch and then an RCBO to power socket circuit in the outbuilding(garage). The garage consumer unit could be configured any way provided it provided RCD protection to the sockets in that outbuilding.

 

Well each MCB has slightly different characteristics.

See when lets say a short happens, the voltage does not instantly just appear. It takes time, all be it millionths of a second but it takes time therefore creates a curve(graph of voltage vs time). Once the MCB has reached its calibrated trip point where it will intersect with the curve created by the fault it would trip. Note that the time to trip for the MCB also takes time so depending on how fast the MCB can trip would depend on how long it would take for the fault curve to reach the second MCB tripping current.

On top of all that you have the energy caused by the fault once the MCB disconnects. This will be dissipated by the spark array inside the MCB which will also cause a spike and may cause another curve that the other MCB may react to.

An MCB uses a bimetallic strip which at a given voltage/current will heat up so much so it pulls a hook and drops the switch.

This is why older MCB's are problematic today. If you change the voltage from what was 250v to 220v the characteristics of the MCB can change. All manufacturers MCB's are different not by much but they are.

And on top of all that Building Control want to make ones life hell for a DIY person because tampering and changing these components requires their approval unless you use a registered electrician.

Where will it end, every time the country needs show some growth in electricians they just change the mains voltage so a rapid demand for repairs to consumer units are needed. lol sorry this is a bit conspiracy but hey, interesting conspiracy would you not say...

 

They are both liable to open. They have an instantaneous operating time of between 0.01 and 0.1 seconds, so realistically a fault current will not allow for Selectivity (discrimination). A fuse of MCCB would be required for this.

 

Right - transients creating a secondary curve?

 

Right Mr R. Must admit my preference for supplying sheds/garages etc is to henly and switch fuse - this feeds the shed etc cu..this way does give the discrimination virtually without fail? as opposed to say running off a cu mcb to second cu in shed etc?

 

Yes.
So many factors when it comes down to the physics of these things which is a bit deep for electrical installation but hey ho it is what it is lol.

See some call 0.01-0.1 second instantaneous. I don't i call that a measurable time.


Oh and i hate nffs and pffs but mffs well

 

Sorry say for arguments sake a 60 amp switch fuse to 40 amp outbuilding setup (all swa)

I mean at the outbuilding end you would expect any mcb tripping (pretend no rcd) to be contained there but maybe not if run off of house cu, but maybe yes if run off of switch fuse? Never thought of this stuff tbqh.

 

Many thanks Fire - never looked at things like that in depth - very interesting though - bit difficult to design things on a technical level though within those time curve parameters, its a bit hit and miss really. I suppose its just best to go with convention. Bill switches rule.

 

You would have to compare the pre-arcing time of the fuse with the operating time of the circuit breaker to ensure full discrimination with the energy let-through values (I^2 * t).

 

Ok - thanks for your time Mr R, its a very interesting topic. Discrimination seems to me to be a bit hit and miss - never really thought about it on a technical level before - I just follow accepted practices. There's a lot more to it really.