Overvoltage and Surge Protection for a house in Malta with 3 phase 63A supply

I want the energy from any potential lightening strike to stay outside my house. Therefore I am considering adding a further earth rod in the front garden near to DB-A which would be used only by the SPD.

No, no, definitely not! The SPDs and earth rod don't keep the overvoltge "outside" at all - rather they momentarily short conductors together to reduce the potential difference between them. Even in a TN system everything will at best hover around at about half the surge voltage, in a TT system where the resistance to Earth is likely to be tens if not hundreds of Ohms so Ohms:Law says there will be an even larger difference between the protected conductors and true Earth.

By way of example, say we had a 50kV surge, originating somewhere upstream of your property, let's say for the sake of argument 0.1 Ohms worth of cable away and your local rod had a resistance to Earth of say 100 Ohms (that's mostly the resistance of the soil etc around the rod of course, rather than the rod itself) and your SPDs were of the theoretically perfect type and had a resistance of 0 Ohms when the activate - the 50kV surge would be divided in the ration of 0.1:100 - resulting in all your wiring (L1/L2/L3/N/PE) at the terminals of the SPD all being at about 49,950V. Equipment inside the installation is protected as it only sees the voltage difference between conductors .. which should be negligible. Connect your Equipment to a different, independent, Earthing facility and now it sees closer to 49,950V between L1/L2/L3/N (at 49,950V) and PE (at 0V).

Have extra rods by all means, but strap them all together and make sure the SPD is at a common point between them and the installation.

Likewise the large currents and large voltages involved mean that considerable voltage differences can be produced along even relatively short lengths of cable - and so you need to ensure that what you're protecting (all conductors, including the c.p.c.) see as little of the surge voltage as possible - 0.5m is the usual maximum for that. As you say, having the Earth originate in the secondary CU (with outgoing circuits connected to it at that point) and a long length to the SPD at the origin would certainly be a bad move.

Don't overlook main bonding of any metallic services or structural steelwork in contact with terra firma either - or again some equipment (esp. that connected to pipework) can see voltage differences that the SPD was meant to hide it from.

My understanding is that MCBs protect the cable whereas RCDs protect people against electric shock.

Sort of. In many systems (TN) MCBs can provide perfectly adequate protection from earth faults (that pose a shock risk) and 30mA RCDs only provide additional protection against electric shock. The statement is less untrue in TT systems where the earth fault current is normally far too low to trigger MCBs, but still it can be useful to distinguish between ADS (i.e. for L-PE faults) which can be provided by almost any rating of RCD and additional protection (e.g. sticking fingers into a lampholder) which requires 30mA (or less) devices.

The main point is that a shock can happen due to an earth fault anywhere in the installation - it doesn't necessarily have to be on a final circuit and RCDs will only provide protection for faults downstream of where they're installed. E.g. a L-PE fault within a CU or the wiring between CUs will raise all the earthing system to nigh on full mains voltage and it'll remain there until something disconnects.  So your options are either to provide something that will disconnect within the required time (1s or 0.2s depending on the detail) - i.e. an RCD - or ensure that earth faults cannot reasonably occur upstream of the RCDs you do have - which means double or reinforced insulation ... which isn't trivial to do. The requirement for protection against electric shock applies to every part of the installation, not just final circuits.

  - Andy.

I want the energy from any potential lightening strike to stay outside my house. Therefore I am considering adding a further earth rod in the front garden near to DB-A which would be used only by the SPD.

No, no, definitely not! The SPDs and earth rod don't keep the overvoltge "outside" at all - rather they momentarily short conductors together to reduce the potential difference between them. Even in a TN system everything will at best hover around at about half the surge voltage, in a TT system where the resistance to Earth is likely to be tens if not hundreds of Ohms so Ohms:Law says there will be an even larger difference between the protected conductors and true Earth.

By way of example, say we had a 50kV surge, originating somewhere upstream of your property, let's say for the sake of argument 0.1 Ohms worth of cable away and your local rod had a resistance to Earth of say 100 Ohms (that's mostly the resistance of the soil etc around the rod of course, rather than the rod itself) and your SPDs were of the theoretically perfect type and had a resistance of 0 Ohms when the activate - the 50kV surge would be divided in the ration of 0.1:100 - resulting in all your wiring (L1/L2/L3/N/PE) at the terminals of the SPD all being at about 49,950V. Equipment inside the installation is protected as it only sees the voltage difference between conductors .. which should be negligible. Connect your Equipment to a different, independent, Earthing facility and now it sees closer to 49,950V between L1/L2/L3/N (at 49,950V) and PE (at 0V).

Have extra rods by all means, but strap them all together and make sure the SPD is at a common point between them and the installation.

Likewise the large currents and large voltages involved mean that considerable voltage differences can be produced along even relatively short lengths of cable - and so you need to ensure that what you're protecting (all conductors, including the c.p.c.) see as little of the surge voltage as possible - 0.5m is the usual maximum for that. As you say, having the Earth originate in the secondary CU (with outgoing circuits connected to it at that point) and a long length to the SPD at the origin would certainly be a bad move.

Don't overlook main bonding of any metallic services or structural steelwork in contact with terra firma either - or again some equipment (esp. that connected to pipework) can see voltage differences that the SPD was meant to hide it from.

My understanding is that MCBs protect the cable whereas RCDs protect people against electric shock.

Sort of. In many systems (TN) MCBs can provide perfectly adequate protection from earth faults (that pose a shock risk) and 30mA RCDs only provide additional protection against electric shock. The statement is less untrue in TT systems where the earth fault current is normally far too low to trigger MCBs, but still it can be useful to distinguish between ADS (i.e. for L-PE faults) which can be provided by almost any rating of RCD and additional protection (e.g. sticking fingers into a lampholder) which requires 30mA (or less) devices.

The main point is that a shock can happen due to an earth fault anywhere in the installation - it doesn't necessarily have to be on a final circuit and RCDs will only provide protection for faults downstream of where they're installed. E.g. a L-PE fault within a CU or the wiring between CUs will raise all the earthing system to nigh on full mains voltage and it'll remain there until something disconnects.  So your options are either to provide something that will disconnect within the required time (1s or 0.2s depending on the detail) - i.e. an RCD - or ensure that earth faults cannot reasonably occur upstream of the RCDs you do have - which means double or reinforced insulation ... which isn't trivial to do. The requirement for protection against electric shock applies to every part of the installation, not just final circuits.

  - Andy.