Flying cables FAQs

Jason Hallam, Technical Specification Manager, CPN-Cudis offers the answers –

What is the fly small functional earth cable for?
This is used in the event of a loss of neutral during an earth fault giving the RCBO reference to earth. At least one incident of loss of neutral supply is reported each day in the UK.

What colour should the functional earth cable be?
In most cases this will be cream/white.

Can I extend functional earth cable?
Yes, this is not a problem.

Can I group the functional earth cables when connecting into the earth box?
This is plausible but we would recommend that they are connected to the earth bar with the corresponding circuit for clear identification.

Can I shorten or lengthen the flying neutral cable?
Yes. If shortened, ensure that the end is correctly terminated – eg with a boot lace ferrule – before connection is made into the neutral bar. If the neutral flying lead is required to be longer, ensure that you select an adequate splicing crimp which is properly insulated, and that the same CSA of cable is used.

Why do some RCBOs have no functional earth?
RCBOs do not have to have a separate earth connection to meet the standard EN61009. The UK seems to be one of a very few countries that have a functional earth facility. Eventually, with the CPN single-pole RCBs, you may see the functional earth facility disappearing.

Can I cut off the functional earth cable?
No, this is bad practice. Even though testing of the RCBOs, as required by BS7671, will be fine, we, as a manufacturer would not recommend this as the RCBO has been type-tested and certified with the functional earth attached.

Do CPN single-pole RCBOs which are used in consumer units and distribution boards disconnect neutral?
No, only live is disconnected, and this applies to most other manufacturers. However, CPN offers a range of RCBOs for all applications.

Can I use single-pole RCBS for caravan parks and marinas?
No, single-pole switching of the live conductor only, and not the neutral, does not meet the requirements as set out in BS7671 Regulation 708.553.1.13 for caravan parks, and Regulation 709.531.2 for marinas and similar locations. When using RCBOs for these applications, a double-pole RCBO, which disconnects live and neutral conductors, must be used. This would also apply to three-phase applications in these locations.

Can I mount RCBOs horizontally?
Yes, this is most common in three-phase distribution boards.

Why doesn’t CPN offer a 50A or 63A RCBO?
CPN is working towards higher current RCBOs. Having a high current rating, in such a compact space, with a neutral supply, has its challenges.

Do I have to disconnect the cables (eg the functional earth and neutral cables) when carrying out an insulation resistance test?
Yes, we need to remember that insulation resistance testing is for the installation only and not the protective device. Subject to voltages used, this could cause damage to the electronic component within the RCBO if not disconnected.

RCBOs are getting smaller and more compact. CPN offers a range of MCB-sized RCBOs for single-phase applications up to 32A, providing you with some welcome space in your enclosures. CPN also offers double-pole RCBOs, which allow the switching of live and neutral up to 32A in B and C curve for applications such as marinas and caravan parks, along with the traditional double pole-sized RCBO.

For more information on CPN-Ducis, visit