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

I notice that the surge arrestor doesn't have a fuse or MCB protecting it.  Does the one you're using have built-in protection?

MOV surge arrestors have a nasty habit of failing short circuit, and need something to disconnect them if that happens.

First off, what's the earthing arrangement? (I've a vague recollection that Malta may use TT and you diagram might be read as suggesting the same).

If it is TT you need to be very careful about the possibility of earth faults upstream of the first RCD - generally everything need to be the equivalent of double or reinforced insulation - which might be tricky if the cabling between DBs contains a c.p.c. (or worse, armour). Also check that the SPD is CT2 rather than CT1 (i.e. has the line SPD elements connected to N rather than PE, with usually a gas-discharge type between N and PE). At least one RCD close to the origin (even if just downstream of the SPD etc) may well have its merits.

UK convention is typically to use a plain switch disconnector rather than an overcurrent protectice device as the incomer in a CU, so relying on upstream overcurrent protection, but that's not universal. In the Irish republic for instance, a B63 is commonly seen as an incomer (or a 63 fuse in older installations) - both approaches have their swings and roundabouts.

  - Andy.

AJJewsbury said:

UK convention is typically to use a plain switch disconnector rather than an overcurrent protectice device as the incomer in a CU, so relying on upstream overcurrent protection, but that's not universal. In the Irish republic for instance, a B63 is commonly seen as an incomer (or a 63 fuse in older installations) - both approaches have their swings and roundabouts.

Those two MCBs are not going to discriminate, but I take the view that tripping should be an exceptional event: if the installation is well-designed and the property is well-maintained, nothing should trip.

I cannot recall anything tripping in our house over the past 28-odd years.

Malta is indeed largely TT, and very dry and rocky (no actual rivers on the island) so decent electrode resistances can actually be quite hard to attain.

It will be worth expecting marginal performance.

Mike

Thanks all for your comments. I am not an expert in these matters, so please bear with me whilst I try to outline the issues:

1. Malta is largely TT (Mike): The electrical system in my house is indeed TT (In Earthed Neutral (TT) System, the neutral from the main transformer is connected to the ground though an earth ground and all metallic parts of the equipment and loads are connected to another earth ground). An earth rod was installed and tested in 2024, but it is close to distribution board DB-X inside the house rather than close to DB-A which is located in a IP65 box in my front garden. This IP65 box also contains the main meter which is connected to the EneMalta cable that runs down the facade.
2. Considerations about the earthing arrangements (Andy): The cable from DB-A to DB-X does contain a Circuit Protective Conductor (CPC) as well as neutral, L1, L2, L3 wires - 5 wires total contained in a non-armoured PVC jacket which complies with local standards. Andy makes a good point about it being unwise to use this CPC wire to provide a conduction path from the Surge Protection Device (SPD) to the earth rod near DB-X as 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. Would this address the issue?
3. Type of SPD - CT1 vs CT2 (Andy): I will specify a CT2 SPD.
4. MCB vs RCD (Andy, Chris): My understanding is that MCBs protect the cable whereas RCDs protect people against electric shock. You can see from the diagram of DB-X that I have employed RCBOs (combine MCB and RCD in one unit) for all the power and lighting circuits in my home fed from the DB-A incomer, so I only need MCBs in DB-A to protect the cables from damage from excess current. The faster tripping time of RDC is irrelevant as a MCB will trip well before the cable is damaged.  
5. Circuit Breaker arrangements (Andy, Chris): The diagram for DB-A shows a MCB protecting the cables downstream from the meter. It also provides a convenient way to isolate the supply to my house. In addition there is a MCB in DB-X connected to the incomer from DB-A which protects the downstream wiring as well as providing a convenient way to isolate the supply from inside my house. Chris seems to suggest that the MCB after the contactor in DB-A is redundant as I had suspected. However, it wouldn't harm to include this MCB as belt and braces as suggested by Andy.
6. Protection of SPD (Simon). The SPD is protected by an upstream MCB so if the SPD goes short circuit then this MCB will trip. There is also a further MCB in the meter box provided by the electricity supplier, EneMalta. 

In summary, I need to fit a further earth rod in my front garden to provide a conduction path from the SPD and then fit the devices shown for DB-A with the SPD specified as type CT2. Clearly, this work will be done by a registered electrician not me.

Again, thank you all for the comments. They are much appreciated. Any further suggestions grateful received.

One further point: The electrician suggested that the rating of the contactor to isolate the supply to the house during over-voltage should be rated at 100A although the MCBs and supply are all rated at 63A. Do I really need 100A rating? 

It will depend on how often it is called to interrupt the full load current. A '63A' or '100A'  contactor will carry 63/100 amps under some controlled condition, usually a well behaved resistive load and at a defined ambient temperature, often 30C,  but it is not intended to interrupt that current without the contacts becoming slowly degraded each  time. If the load is reactive (inductors - think motors, or capacitors, think electronics and some kinds of light fitting) the surge currents may be much higher than the average, and some de-rating is required or the contacts get burnt up unusually fast.

The use of the next size up means that concerns about longevity and or a warmer environment are mitigated, and avoids having to think hard about the location or extra ventilation. The electrician may also be thinking about avoiding call backs if he or she has seen problems in the past. If upgrading at design time the price increase is usually modest, if a model is selected where both ratings are for the same frame size, as it is essentially beefier contacts in the same box.
Mike.

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.