RCD on TT supply

[Mecca]

Hi all,
This may be an easy one to answer, but might be hard to explain...

I maintain a residential property which relies on three flood water pumps to avoid being flooded when the water table is high. The electrician has fitted an ATS generator. As the building is remote from the main house which is PME, an earth rod has been fitted. The electrician has fitted a 32A 30mA RCBO in the fuse box to supply the ATS unit. The ATS then feeds three 6A RCBOs for the pumps and one 16A RCBO for a socket and light in the generator shed.
The problem I have is that if any one pump has a fault, it is likely to trip the main 32A RCBO which would flood the building. There are voltage and float alarms and such like, but no good if no one is in.
To get to the point, can I change the 32A RCBO which supplies the ATS to an MCB?
The cable is in a 3m length of conduit which doesn't run in any walls and the ATS would be the only device not RCD protected and it immediately feeds the other RCBOs.
Is this allowed with a TT system? If not, which level of RCD protection is the minimum required (eg time lag, higher current?). Ideally we would want no RCD between ATS and CU, but the electrician said this is not possible.

Any advice appreciated

 

[ericmark]

The description does not seem to make sense, you have an automatic transfer unit and a generator in a out building with TT earth feeding a house with a TN-C-S supply? Some thing missing.

The RCD with a TT system is the primary protective device, and with a TN-C-S it is secondary, so with a TT system the type matters, but with TN-C-S not so critical, but there is a list BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436 of cable types which can be buried without RCD protection, if not buried then it is dependent on if sockets etc are fitted, however some one has to sign for the design, so the only person who can say yes or no is the person signing the certificate.

There are auto resetting RCD's but not normal with domestic, there are also 100 mA and 300 mA RCD's, there is also a residual current monitor (RCM) but clearly some one has to do a risk assessment.

313.2 Supplies for safety services and standby systems
Where the provision of safety services is required, for example, by the authorities concerned with fire precautions and other conditions for emergency evacuation of the premises, and/or where the provision of standby supplies is required by the person specifying the installation, the characteristics of the source or sources of supply for safety services and/or standby systems shall be separately assessed. Such supplies shall have adequate capacity, reliability and rating and appropriate changeover time for the operation specified.

What it says in the 2018 edition I don't know, but to stop flooding is a special case, and not as simple as saying must be RCD protected.

However no one can really advise without being on site, and it would need paperwork to cover the work.

 

[Mecca]

Thanks. To clarify, the main house is TNCS. A 40A submain from the main house feeds this secondary residential outbuilding which is not connected to the earth of the submain cable but instead has its own earth rod.
The generator is just for the pumps, which are sited next to the outbuilding which has the flooding. Nothing to do with the main house.

 

[ericmark]

I would say if the whole run of the 40 amp submain is SWA then no need for a RCD at origin, however if it goes through the house as twin and earth and only becomes SWA after it leaves the house, then needs RCD protection.

 

[davelx]

I would say if the whole run of the 40 amp submain is SWA then no need for a RCD at origin, however if it goes through the house as twin and earth and only becomes SWA after it leaves the house, then needs RCD protection.

It may need RCD protection, or it may not. It depends on how and where the T&E cable is run within the house. For example, it could be sitting on a concrete pad 30cm below a suspended ground floor, and then emerge vertically through the floor beneath the CU, be clipped to the wall and run into the house CU. No need for RCD protection in that situation.

 

[Mecca]

Sorry, the RCD is not on the SWA. It goes:
Main house is on TNCS 100A supply phase 1.
40 switch fuse is on phase 2, no RCD, which feeds the SWA going to the CU in the outbuilding. The SWA goes straight into the CU The earthed armour is not connected to this CU. This CU has its own rod earth supply, and the ATS comes off this CU using a 32A RCBO. Hope that's a bit clearer.
Just want to eliminate this RCD for the ATS if allowable, or fit time delay (?mS, ?mA).

 

[Mecca]

The cable from the 32A RCBO to the ATS is 4mm H07 ducted straight through the outside wall (it's a basement) into the ground and ducting comes up inside generator shed and straight into the ATS. 3m run max.

 

[davelx]

Then it depends on how the cable is run from the CU in the outbuilding to the ATS.

The ATS itself does not need RCD protection.

 

[Mecca]

Thanks davelx, see above

 

[Mecca]

I'm guessing from the replies that maybe if the cable to the ATS had been armoured or in earthed ducting then we could eliminate the first RCD?

 

[davelx]

Armoured or in earthed conduit/ducting or surface mounted or buried deeper than 50mm in a wall.

 

[333rocky333]

You dont mention how the other circuits in the outbuilding are Rcd protected
Is there an Rcd protecting the whole board.

Surely, you still need an Rcd for fault protection of the Generator room supply cable more so with a new SWA, as well as if the H07 Flex stays, as its connected to a TT system.

I think you can omit an Rcd for maybe a short length, if protected and Double insulated, but not sure about 3 metres underground, maybe a 100ma S type Time delay Rcd will discriminate with your 30ma Rcbo's

Reg531.3.5.3.2.201 although mainly for Metal Cu's, i quess would apply in your situation

 

[Mecca]

You dont mention how the other circuits in the outbuilding are Rcd protected
Is there an Rcd protecting the whole board.

There is a separate CU in the outbuilding for lights, power etc, all on their own RCBOs.

 

[Mecca]

Reg531.3.5.3.2.201 although mainly for Metal Cu's, i quess would apply in your situation

I'm not sure this does apply, as there is a cable from the CU to the ATS and then to the downstream RCBOs. I think having read these posts, that a time delay RCD is the way to go. After all I'm simply trying to avoid one faulty pump from tripping the whole lot (which defeats the object of giving each pump an RCBO).
Thanks for your help

 

[333rocky333]

I'm not sure this does apply, as there is a cable from the CU to the ATS and then to the downstream RCBOs.

The Rcbos do not provide fault protection to that cable, earlier posts suggested if Swa cable or earthed containment was used, that it would allow you to remove the RCD', however a fault on that cable would not trip an Mcb leaving the metal swa glands and any metalwork at a dangerous voltage