Solar PV Inverter Circuits and the possible need for RCD Protection

RCD’s and RCBO’s as we know are excellent devices for protection against shock (known as additional protection) and line earth faults (known as fault protection). RCD’s are also used to provide an early warning of insulation break down to prevent a fire. The question is are they necessary to cover Solar PV circuits? Having said that, generally there are only a couple of instances where they will be required in a domestic property by BS 7671:2008, for example, if the wiring method is a Twin & Earth cable buried in the fabric of the wall less than 50mm deep (additional protection) or if the building is fed from a TT system (fault protection). Other than that there is no specific requirement to protect Solar pv circuits with RCD/RCBO protection. In fact, I advise installers to avoid RCD protection altogether if possible, especially using a 30mA device, as many inverters are now electronic and leak to earth by nature, which will make the circuit prone to nuisance tripping.

To negate the use of an RCD/RCBO on Solar PV circuits in terms of burying a cable in a wall, I would suggest the use of a Flexi Shield cable to BS 8436 or a cable which has an earthed metallic sheath under Clause (i) Reg 522.6.101 on pg 124, or one of the other options clause (ii to iv). To avoid the requirement for RCD protection altogether Reg 522.6.102 pg 125, simply running the cable on the surface in robust wiring system or a containment system such as plastic conduit or trunking, would be a solution.

If it is not possible to avoid the use of RCD protection on say a TT system (fault protection), then as Reg 712.411.3.2.1.2 on pg 242 suggests a Type B (ideally a 100 mA device - although it doesn’t say that) would be necessary, as a leakage current to earth due to a fault downstream from the device may not be sinusoidal in nature. If the inverter does not have simple separation ie a transformer (possibly galvanic isolation) or some other fail safe technology (Ref the manufacturer), then an earth leakage current may be pulsed or pure DC in nature, which a general Type AC RCD will not pick-up and thus fail to operate. This is why the aforementioned Reg requires a Type B device is installed, which will sense AC, half wave DC and pure DC leakage currents. The different types of RCD are very often denoted by a symbol in a box on the front of the device. Incidentally, the Type B device is hard to obtain, is relatively expensive, and the installer will require a test instrument that can perform a Type B RCD test. In addition, RCD’s and RCBO’s will have to be double pole devices to avoid the situation shown on page 14 of the ESC Best Practice guide.

In terms of good practice, if an installer cannot avoid using an RCD/RCBO protection, or even without - the preferred option is to install a separate DB dedicated only to the solar PV circuit. By doing this the installer will keep any problems with either the general consumer unit or PV supply DB completely independent of each other. Of course this does not rule out any other options, but to my mind this is the ideal solution and would also provide a safer isolation procedure for maintenance/servicing in terms of two supplies coming into one DB.