RCDs/RCBOs in series

Why does the sub-main require RCD protection?

 

S type at the origin if TT, Sub can be SWA with standard breaker on it if TN and you can get your times down

You sound a little out your depth Mr Awesome, this is electricians talk so I guess you are one.?

 

MCB not RCD at house end, SWA cable no rcd reqd. TT garage end earth rod and RCD , jobs a goodun.

 

Currently not mandatory yet but sometime in the future it will be.
The EU want to get rid of the split consumer unit and have every circuit after the main MCB going into RCBO's. Just putting that out there...

 

Well they will have to be quick, we'll be out by this time next year.

 

Yes that is so undoubtedly true.

 

What is in the op that makes you think it requires an earth rod?

 

Ok so this is getting complicated when it does not need to be.

If you are going to install a mini consumer unit in each one for the shed and another one for the garage.

You would install each consumer unit which will have a main breaker followed by a set of RCBO's for each circuit in the garage to that consumer unit.
That is one RCBO for the garage and another RCBO for the shed.
That would then travel on a high current armoured cable from the house consumer unit to each external rooms.

If you are worried about this RCBO in the house tripping due to what you do in the garage or shed, tough. It is there for a reason to stop your house burning down and burning your family alive within it.

http://electrical.theiet.org/wiring-matters/16/elect-inst-outdoors.cfm?type=pdf

Just feed it from an RCBO, that way if you trip it it will only affect that RCBO and not the rest of the house circuit.

Why would you trip it?

Do you plan on some kind of electric shock erotica?

You could install a 10ma RCD in the mini consumer unit in the shed and garage that way this RCD will trip well before the residual current would in the house.

Sorry you can not use anything higher than a 30ma RCBO for exterior use. so it has to be 30ma or less.

You need to know what protective circuit you have to correctly install this.
The armoured cable run you feed to the out buildings must carry an earth wire from the house consumer unit and must be correctly tested.

Also sorry about the bad news but you also need to replace the house consumer unit if you do want to do this. As of January 2016 with Amendment 3 of the regulation the consumer unit needs to be updated to a NON combustible housing.
Plastic consumer units just don't cut it any more.
They have proven to catch fire because of bad workmanship particularly not correctly torquing up the connectors or the terminal clamp getting jammed and not clamping the cable properly.

 

Install a new garage box in the house with no RCD then one in the shed with an RCD if you've no non RCD ways in the existing one.

 

There is so much wrong with this post it is almost funny. Far too late to deal with it now.

 

Why does the SWA need RCD protection and why does he need to replace his house consumer unit? The regs are not retrospectively applied

 

Regs say any electrical circuit going outside needs RCD protection.

Amendment 3 recognises plastic consumer units catching fire so the consumer units housing is to be non combustible as to contain any ignition source and stop it from spreading.
Unit must be changed if any new addition to a circuit is added or overhaul work is carried out. A DIYer could get away with this but the problem is still relevant.

I know what you are saying, how could a SWA cable become damaged to such a point that an RCD would even be needed. Maybe regs wording needs changing then to accommodate your observation lol.

 

The circuits would be covered by the RCD in the garage.
If a new garage unit was fitted next to the existing consumer unit then I don't see the need for the consumer unit to be changed. no work is being done on it and no circuits are being added to it.

 

You are saying to be putting a mini consumer unit in the house next to the house consumers unit and and run a couple of tails to it from the main feed which you will need to splice into and junction off?
This would jump past the houses consumer unit completely and be a separate unit from any and all current circuits. Interesting idea but the mini consumer unit will need a main MCB and still at least one RCBO for each outbuilding before the circuit leaves the house.

The current rating of the MCB and RCBO will be defined by the cable characteristics used which would be designed around what kind of circuits will be wired in the outbuildings.

 

There is a very good reason for having an RCD before the circuit leaves the building.
In the event that current around a circuit somehow disappears greater tha 30ma this means the energy is not being used by an appliance but somehow dissipates in heat or some other form of odd way.
an RCD detects this and say hell no.



So regulations are made as so regardless of what you do, any circuit leaving the building needs to be fed from an RCD to protect from anything that may happen.

MCB's protect circuits
RCD's protect your life.

 

Fire think you need to study up a bit!
Heat is a natural output of using energy and is not connected with the tripping of RCDs.
RCD's have a core balanced CT if the current going down the live is not balanced with the current returning in the Neutral the CT produced a current in the secondary winding. If this gets high enough the RCD will trip. In this application somewhere above 50% of its rating, so just north of 15mA, but absolutely less than 30mA.

 

This is so eye watering I can barely see. . .

 

Any new CU to be installed will need to be NC. Adding new circuits to an existing CU does not mean you need to replace the whole board.

Also the SWA does not need RCD protection and not all external circuits require RCD protection either.

Do you own a copy of BS7671?