5.6: Coordination Between Conductors & Overload Protective Devices
The relationship between conductors and overload protective devices is a critical part of electrical safety within an installation.
Checking coordination between conductors and overload protective devices involves determining if the cable installed is adequately protected against overload and is safe for continued usage.
Whilst it may seem cut and dry on the surface (IE: applying cable carrying capacities), there are actually a couple more factors which need to be considered to ensure that the setup is correct.
The coordination depends on the following factors:
- Current Carrying Capacity of Cable Used
- ** Don’t forget to apply correction factors for installation method **
- Potential Ring Continuity (if circuit in question is a Ring Final Circuit)
- Correction factors for different overload protection devices (specifically BS3036 fuses)
Lets look at each of the factors in slightly more detail below:
Current Carrying Capacity Of Cable Used
The main factor which is quite obvious really, is whether the current carrying capacity of the cable being used is suitable for the both the load encountered, and whether or not the chosen overload protective device will actually do it’s job of protecting the cable.
So a well designed circuit will have a load which is lower than the overload protective device to be used for that circuit. The cable to be used will then have a current carrying capacity which exceeds both the load, and the rated current of the overload protection device
In this method, the circuit load will never exceed the current carrying capacity of the cable. Should there be an overload for some reason (usually a fault), the overload protective device would be rated such that it still protects the current from exceeding that which the cable can safely carry.
In a nutshell, this is the coordination between conductors & overload protective devices.
When completing an EICR (Electrical Inspection Condition Report), the cables should be checked against the relevant current carrying capacity tables within Appendix 4 of BS7671.
However, that’s just the first step. Don’t forget to check (and if nessecary) apply the correction factors which might apply depending on how the particular cable is installed:
Correction Factors For Cable Installation Method
Coordination between conductors & overload protective devices can be more difficult that just cross referencing the tables within Appendix 4.
Towards the front of Appendix 4 (before the actual tables) is a section detailing a large number of ‘correction factors’ which must be applied to the current carrying capacities if the particular cable is installed within a specific way.
A small number of examples (although by no means exhaustive) are:
- Cables installed within thermal insulating material (IE: through loft or wall insulation)
- Cables which are to be used within areas with higher than normal temperatures
- Cables which are installed within ‘bunches’ of other circuits
- Cables buried within soils of different thermal resisitivity
These correction factors are often overlooked, particularly during periodic inspection.
It can often be difficult, indeed sometimes impossible to check every section of cable to check whether or not it meets these conditions, however it is important to bear them in mind in situations where cables are quite obviously heavily grouped together or ran through (as opposed to over or under) thermal insulation.
Ring Continuity (if testing Ring Final Circuits)
It may seem obvious, but if a socket ring final circuit does not have correct ring continuity, then it will not have proper coordination between the conductors & overload protective device.
For example, a 32A socket ring final circuit is normally wired with 2.5mm T&E cable. The cable only has the ability to safely carry upto 32A due to the fact that it is wired as a ring final circuit.
If there is a break in the continuity at any point, the circuit effectively becomes 2 radial legs fed from the 32A overcurrect protective device (be it an MCB or RCBO). In this instance the 2.5mm cable is only rated for 27A (if the installation method is C: Clipped Direct) and hence the cable would not be adequately protected against overload.
This also applies in instances where more than 1 spur has been made from a single point on a ring final circuit. Appendix 15 gives instructions on how to properly apply spurs to a ring final circuit. Unfused spurs can only be one single or double socket at any single point on the ring. Spurs which are fed from a fused connection unit (also called a fused spur) can support more sockets as the cable is protected by the 13A fuse within the FCU.
Correction Factors for Different Overload Protective Devices
This particular issue mainly revolves around the use of BS3036 semi-enclosed fuses as overload protective devices.
Due to their nature, the can allow higher levels of current to flow before ‘blowing’ (which is really the correct term for these as the fuse element literally melts).
Because of this, a correction factor of 0.725 must be applied to any cable current carrying capacity where the cable is protected by a BS3036 fuse.
In this instance, a clipped direct 2.5mm T&E which previously had a CCC of 27A becomes 19.58A (27 x 0.725 = 19.575A).
A particular one to watch with this is 1.5mm T&E radial circuits fed by a 15A BS3036 fuse. With a 15A (or even 16A) MCB or RCBO, this would be perfectly acceptable, however with the correction factor applied, 1.5mm T&E is only suitable for 14.5A and hence does not have the correct coordination between conductors & overload protective devices.
EICR Coding For Conductor & Overload Protective Device Coordination
This one is quite simple.
There either is correct coordination between conductors and overload protective devices, in which case no code is required on an EICR
Or, there is no correct coordination between conductors and overload protective devices. In this instance, the only appropriate code is a C2 – POTENTIALLY DANGEROUS
So if the cable is not adequately protected by the MCB, RCBO or fuse, or a socket ring final circuit does not have the correct continuity then this would warrant the EICR being UNSATISFACTORY