Firefighter PV isolation

What remains energised during a PV emergency, and how design, labels and the building fire strategy support responders.

Solar modules generate DC whenever enough light reaches them. Turning off the consumer-unit circuit or AC isolator stops the inverter feeding the AC installation, but it does not de-energise the array or the DC cables between the modules and their isolation point.

That distinction shapes fire-service information and cable routing. A label saying “solar isolated” must not imply that the roof is electrically dead when it is not.

What each control actually does

Main switch or PV circuit-breaker. Disconnects defined AC circuits. Other sources, backup circuits and the PV DC side may remain live.

PV AC isolator. Separates the inverter’s AC connection. The inverter should cease grid-connected operation, but illuminated modules and upstream DC wiring remain energised.

PV DC switch-disconnector. Isolates the inverter from the array at that point. The modules and cable between array and switch remain live in daylight.

Module-level shutdown or controlled-voltage system. Can change the voltage on defined conductors when correctly triggered, but its behaviour depends on compatible modules, electronics, control power and product design. It is not assumed from the presence of optimisers.

Firefighter’s switch. A defined device used as part of a building fire strategy for specified equipment. The current BS 7671 approach places firefighter’s switches where the fire engineer identifies them as part of that strategy. It does not create a blanket domestic-PV rapid-shutdown requirement.

The UK rapid-shutdown gap

UK domestic PV rules do not generally require the US-style rapid shutdown of every conductor on the roof. Some systems offer module-level voltage reduction, but it is a designed product function rather than a general consequence of opening an AC switch.

This makes passive measures important:

keep DC cable routes outside the building where practicable
minimise concealed DC runs through occupied or protected spaces
protect cables from mechanical damage, heat and pests
separate and identify PV wiring from other services
position isolation where it is safe and useful for maintenance
provide an accurate plan for responders and later electricians

Where a building’s fire strategy calls for active shutdown, the specification should define the achieved voltage, affected cable sections, operating control, fail-safe behaviour, signage and test regime.

Labels and plans

The intake and relevant distribution positions should warn that the installation has PV and may have more than one source. Labels need to remain accurate after batteries, optimisers, microinverters or backup are added.

The handover information should show:

array positions and module-level equipment
DC cable routes, including concealed sections
inverter, combiner, battery and isolation locations
AC and DC boundaries for each control
any shutdown device and what remains energised after operation
contact and emergency information required by the system design

A generic sticker is not a substitute for a site diagram. Firefighters may arrive when the owner, installer and cloud app are unavailable.

Isolator placement is not firefighter protection by itself

Adding a roof DC isolator can increase weather exposure and the number of DC connections. It also remains beyond safe reach during many incidents. Its presence should be justified by the electrical maintenance design, not a claim that emergency crews will climb up and use it.

An accessible DC isolator beside a ground-level inverter makes the inverter side safe after operation, but the array-side cable remains energised. Its label should say so clearly.

No householder should approach an inverter, battery or rooftop isolator during a fire. Leave the property, call 999 and tell the operator that solar PV and any battery storage are present.

Fire risk and current research

Government-commissioned investigations found that PV fires were uncommon but identified preventable problems including incorrectly selected DC isolators, poor terminations, incompatible connectors and water ingress. More recent Building Safety Regulator research has examined how panels can alter the heat and fire-spread conditions above roofs, including concerns for some integrated-PV constructions.

The newer research informs ongoing guidance development; it is not a reason to publish a universal setback or rapid-shutdown rule that current standards do not contain. The installation must preserve the roof’s required fire performance and use the current building guidance for its nation and building type.

Existing installations

Arrange a competent inspection where:

no labels or system diagram can be found
DC cables run through the building with no documented route
isolators are damaged, hot, discoloured, wet or difficult to operate
more than one cable passes through an unsuitable gland
connectors of unknown or mixed manufacture have been used
batteries or backup have been added without updating the diagram
emergency controls are described by marketing terms but their electrical boundary is unclear

Do not operate a suspect DC isolator under load. Its safe inspection may require the inverter to be shut down first and the device operated off-load under a controlled procedure.

Isolation design and documentation

A good design states what firefighters and maintainers can isolate, what remains live, how DC cables are routed and which documents will be left at the supply position. If it promises “complete shutdown”, it names the product standard, trigger, resulting voltage and cable sections covered.