AC isolator placement

How inverter AC circuits are isolated for maintenance, including systems with storage and backup.

An AC isolator disconnects an inverter or other generating unit from the AC installation so it can be worked on safely. It does not make the solar array dead, and in a battery system it may isolate only one of several energy sources.

The design goal is safe, clearly identified isolation at every source and point of work. That does not always require a separate red rotary switch beside each inverter.

What can provide AC isolation

Depending on the installation and equipment, isolation may be provided by a suitable switch-disconnector, circuit-breaker, RCBO or another device with an isolation function recognised by BS 7671. A protective device that trips on fault is not automatically suitable for isolation, and a switch that looks like an isolator is not automatically rated for the duty.

The selected device must suit:

system voltage, number of phases and maximum current
bidirectional current where generation or storage can feed through it
the fault level and required making and breaking duty
the conductors that the earthing and source arrangement requires it to disconnect
enclosure, temperature, moisture and other external influences
the consumer-unit or switchgear manufacturer’s assembly rules

A double-pole rotary switch is common on single-phase systems, but “double pole everywhere” is not a substitute for deciding how neutral is treated in TN, TT and island-mode arrangements.

Position and access

The isolating device should be readily identifiable, accessible for the person maintaining the equipment and associated unambiguously with the inverter or circuit it controls.

Where the inverter is close to the distribution board, a suitable, lockable device at the board may provide the necessary isolation. Where it is remote, out of sight or in another building, local isolation may be needed so a person working on the inverter can control the device and prevent reconnection.

Manufacturer instructions can require a local device or define the permitted switching sequence. The designer should also consider:

whether the device can be locked off
whether another person could mistake or reset it
safe access without entering a loft or exposed location unnecessarily
whether it remains reachable during maintenance or an equipment fault
labels at both ends of a long or unfamiliar circuit

An isolator hidden behind the inverter, inside locked furniture or beyond the equipment it is meant to make safe has not solved the access problem.

One device or several

A single device can isolate more than one unit only when its rating, circuit arrangement and labelling make the result safe and clear. Multiple inverters, different phases, a battery inverter and an AC-coupled solar inverter can create more than one AC source.

The single-line diagram should show every source, point of connection and isolation device. Before work starts, the electrician should be able to prove which conductors can still be energised from:

the public supply
PV generation
battery storage
an EPS or island-mode output
another generator

Turning off the main switch may not remove all of them.

Battery backup and changeover equipment

A backup gateway changes both the source and the relevant isolation boundary. The system may need separate devices for the grid input, inverter, backed-up distribution and non-backed-up loads, plus a safe bypass arrangement.

Neutral-to-earth switching and source separation must not be defeated by operating or adding an AC isolator. The labels and diagram should make connected mode, island mode, maintenance isolation and bypass distinct.

An inverter’s grid-connection protection is not a maintenance isolator unless its declared construction and the complete design establish that function.

Operating sequence

Follow the manufacturer shutdown and restart sequence. A typical process reduces or stops inverter operation before opening isolators, but the correct order depends on the equipment and whether a battery or backup output is present.

Do not treat the AC device as proof that the DC side is dead. PV strings can remain at full open-circuit voltage in daylight after the AC circuit is isolated. Battery terminals can remain energised until their own approved isolation procedure is completed.

The owner should know which device provides routine emergency isolation, but internal testing and lock-off are for a competent person.

Fire and emergency use

AC isolation prevents an inverter feeding the AC installation or grid through that connection. It does not remove voltage from modules or DC cables between the array and inverter.

A switch should not be labelled as a firefighter or rapid-shutdown device unless it performs the defined function in the building’s fire strategy. Domestic PV normally relies on clear identification, sensible cable routing and emergency-service procedures rather than assuming an AC switch de-energises the roof.

Handover evidence

The handover pack should include:

the device make, model, poles and ratings
its isolation and switching declaration
a single-line diagram and physical location
identification of every other possible source
lock-off or safe-isolation instructions
the manufacturer shutdown and restart sequence
test results and labels fitted

If the drawing cannot show what remains live after each device is opened, the isolation design is not clear enough.