13.2 AC Isolation and G99 Relays

Once the inverter has a legal home, the AC side must still be capable of being made dead for maintenance and of disconnecting from a distressed grid. City & Guilds 2922 handbook v1.3 splits that work across 4.4.1 Isolation and switching and 4.4.4 G99 Relay. Do not treat them as the same device. Isolators are for people. The G99 relay is for the network when the generating capacity is large enough to sit outside typical G98 type-tested microgeneration.

Isolation between inverter and AC supply

Handbook 4.4.1(a) requires proper isolation between the inverter and the AC supply, switching all live conductors. On a single-phase domestic circuit that means line and neutral — a double-pole isolator, not a single-pole 'lighting switch' on the line only. On three-phase plant, every line conductor used by the inverter is isolated; if a neutral is present in the circuit, it is included in the isolation scheme rather than left connected as a surprise return path.

All live conductors is the phrase examiners use to catch single-pole shortcuts. A person working on the inverter AC terminals must not find a connected neutral that is still referenced to the installation. Functional switching that merely stops generation is not isolation.

Accessible for general maintenance

Isolators must be accessible for general maintenance (4.4.1(b)). Accessible means an electrically skilled or instructed person can operate the handle without dismantling furniture, emptying a loft of stored belongings, or standing on an unsafe joist. The device is part of the maintenance method statement: isolate, lock-off, prove dead. If you cannot reach the handle, you do not have isolation in the handbook sense.

Securely locked in the off position

The isolator must have the capability to be securely locked in the off position (4.4.1(c)). Lockable off is the requirement; lockable on would be a hazard. Use a device with a padlock facility or an equivalent lock-off hasp so that the inverter cannot be re-energised while someone is on the AC terminals. Combine this with the DC isolator lock-off from the DC chapter: both sources, both locks, then prove dead.

Bidirectional isolator and protective devices

4.4.1(d) requires the isolator and protective device(s) to be bidirectional. A PV AC circuit can carry current from the public supply toward the inverter and from the inverter toward the installation and the grid. A fault on the consumer-unit side of the PV overcurrent device can be fed by the inverter. A grid-side fault can be fed through the inverter from the array. Devices that are only characterised for one direction of current, or that are not rated to interrupt current from the generator, are the wrong product. Select AC isolators and MCBs/RCBOs that the manufacturer rates for generator / bidirectional use at the inverter's maximum AC current.

That same idea reappears under regulation 551.7.2 when generator sets operate in parallel with the prosumer's supply (section 13.3): cables and enclosures can be energised from more than one source, so overcurrent protection has to be thought through in both directions, not only as a 'final circuit from the board'.

Additional isolator when the inverter is in another room

4.4.1(e) is the layout rule that produces extra hardware on real jobs: an additional isolator, situated adjacent to the inverter if it is installed in a different room from the main isolator. The main isolator is typically at the consumer unit or designated point of isolation on the PV AC circuit. If the inverter lives in a loft, garage, or plant room while that main isolator is in the hall, you fit a second AC isolator next to the inverter.

Adjacent means usable while you are working on the inverter, not 'somewhere on the same floor'. The extra device still switches all live conductors, is lockable off, is bidirectional, and is labelled under 4.5 (section 13.4). It is not a substitute for the main isolator; it is in addition.

G99 relays — what they are and when they appear

A G99 relay (4.4.4) is a monitoring device that controls a contactor or other isolating means. It monitors grid voltage and frequency and disconnects the inverter from the grid should a fault occur affecting voltage or frequency. It is typically only required on larger systems over 16 A per phase.

Engineering Recommendation G98 covers fully type-tested microgenerators up to and including 16 A per phase. A typical domestic string inverter in that band includes integral interface protection: loss of mains, under/over voltage, under/over frequency. You still provide the 4.4.1 isolators. You do not add a separate G99 relay just because the unit is called an inverter.

G99 applies where generation exceeds 16 A per phase (MCS also reminds designers that the threshold is aggregated: a 3 kW PV inverter plus a 3 kW battery on the same single-phase supply can push the installation over 16 A even if each box looks 'domestic'). On those larger or non-type-tested arrangements, interface protection may sit in an external relay that trips a contactor, isolating the generator from the grid when V or f leaves the allowed envelope.

Do not confuse the G99 relay with:

• the AC isolator (manual, lockable, for people)
• the DC switch-disconnector (array side)
• a Type B RCD (residual current, not grid V/f)

The relay watches the grid. The contactor is the isolating means it controls. If the paper asks what a G99 relay does, answer with monitoring plus disconnection on V or f fault, typically above 16 A per phase.

Domestic G98 string inverter versus systems over 16 A (G99)

Scenario: inverter in the loft, consumer unit in the hall

The PV AC circuit originates at a dedicated outgoing way in the hall consumer unit. That is the natural place for the main AC isolator and the bidirectional overcurrent device. The inverter is in the loft — a different room. Handbook 4.4.1(e) therefore requires an additional isolator adjacent to the inverter.

Worked sequence for a competent person replacing the inverter:

1. Operate the loft AC isolator (all live conductors) and lock it off.
2. Operate the DC switch-disconnector and lock it off.
3. Confirm the hall isolator is also off and locked if your procedure isolates both ends of the AC cable (two sources can energise that cable).
4. Prove dead on AC and DC before glands come off.

For emergency isolation at the origin, the hall device is the point people can reach without entering the loft. For work at the inverter, the adjacent loft device is what 4.4.1(e) exists to provide. One isolator in the hall and none in the loft is a 4.4.1 fail on this layout. A G99 relay is not required merely because the inverter is in the loft; loft versus hall is an isolation-layout question, not a 16 A question.