DC isolator placement

How PV DC switch-disconnectors are selected, configured and positioned without adding avoidable fire risk.

A PV DC isolator is a switch-disconnector used to isolate the inverter from one or more solar strings for maintenance. It has to interrupt DC safely at the array’s worst credible voltage and current.

DC switching is less forgiving than AC switching because the arc does not benefit from a regular current zero. A wrongly rated, wrongly configured or badly terminated device can arc and overheat rather than isolate safely.

Select from the array design

The nameplate values at standard test conditions are not the final equipment ratings. The designer calculates maximum open-circuit voltage for low module temperature and maximum short-circuit current for high irradiance and parallel strings.

Current IET guidance uses the BS 7671 design factors:

maximum string open-circuit voltage: number of series modules × module Voc at STC × 1.2
maximum circuit short-circuit current: number of parallel strings × module Isc at STC × 1.25

The selected switch-disconnector then has to meet or exceed those design values in its actual pole configuration. Its rating can fall as DC voltage rises, so a current number printed on the front is not enough.

For a conventional external device, look for conformity with BS EN IEC 60947-3 and the correct PV utilisation category. DC-PV0 is a disconnector without on-load breaking duty. DC-PV1 can suit defined single-string duties, while DC-PV2 covers duties where significant overcurrent or reverse current can occur, including relevant parallel-string arrangements.

Follow the manufacturer’s connection diagram exactly. Series and parallel pole links determine the safe voltage and breaking current; changing them changes the device rating.

Integrated or external

An inverter can include a compliant DC switch-disconnector. If it provides the required isolation for the intended maintenance, another external device beside it may add terminals, enclosures and failure points without improving safety.

An external device is appropriate where the design needs a separate controllable isolation point, the integrated device does not provide the required function, strings are combined elsewhere, or manufacturer instructions require it.

The specification should say what each isolator makes safe. “Extra isolation” is not a benefit if no one can identify the live parts left on each side.

Roof and array-side isolators

A roof-mounted DC isolator is not a general UK domestic requirement and is not rapid shutdown. It remains exposed to weather, heat and difficult access, and it leaves the modules and short cable sections energised in daylight.

Putting a device by the array can be justified for a defined maintenance or system architecture, but the designer must account for environmental exposure, access, cable routing and the additional connections. It must not be presented as allowing firefighters to “turn the panels off” unless the actual isolation boundary supports that claim.

For an inverter at ground level, a correctly selected integrated or adjacent isolator often gives the maintainer a controllable point without placing a switch on the roof. The DC cable between array and isolator still has to be routed and protected as live PV cable.

Installation details that prevent failures

Government fire investigations found recurring DC-isolator problems involving incorrect ratings, poor wiring and water entering badly installed enclosures. The installation should therefore verify:

each gland is suitable for the cable count, shape and diameter
entries preserve the enclosure rating and avoid collecting water
conductors and ferrules suit the terminals
strip length and terminal torque follow the manufacturer
no conductor is strained or sharply bent at the terminal
polarity and pole configuration match the diagram
the enclosure is protected from foreseeable heat, sunlight, impact and moisture
connectors and isolators are accessible for inspection

Do not pass two separate cables through a gland designed for one. Avoid upward-facing entries where water can collect unless the product and sealing arrangement are specifically designed for them.

Never unplug a PV connector to interrupt load current. A connector is not a switch-disconnector and can sustain a dangerous arc if separated under load.

Microinverters and module electronics

With microinverters, each module normally connects to nearby conversion equipment, so there is no long high-voltage DC string to a central inverter. AC isolation and the manufacturer’s module-level disconnection arrangement replace the conventional central DC-isolator layout.

The module leads can still produce DC in daylight, and the roof contains energised equipment. Optimisers can change voltage behaviour but do not create a universal safe voltage unless the complete system is designed, powered and operating as declared.

Inspection and maintenance

PV is not fit and forget. Periodic inspection should compare the device with the array records and check rating, pole configuration, enclosure condition, water ingress, heat damage, terminal condition and operation using a safe off-load procedure.

Thermography can help under suitable generation conditions, but it does not replace physical inspection or rating verification. A cool undersized device on a dull day is not proved safe.

The handover pack should include the string schedule, calculated maximum voltage and current, isolator datasheet, connection configuration, cable and connector types, torque records and a drawing showing what remains energised on each side.