Fire safety and DC arc risk on a factory rooftop array

By Vvon Engineering Team | Published 2026-03-26 | Updated 2026-09-13

Understand daylight DC hazards, connector compatibility, isolation and shutdown scope, with questions for the responsible electrical and fire-safety designers.

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The electrical safety file in a Bangladeshi factory is written around alternating current. Fuses, MCCBs, earthing, the Fire Service inspection for the fire licence, the electrical checklist an RMG building goes through for the RSC. A rooftop solar plant adds several hundred volts of direct current running the length of the roof, live whenever there is light on the glass, that no switch in the building can turn off. It is a different hazard and it needs its own set of questions.

A DC arc does not put itself out

An alternating current at 50 Hz passes through zero a hundred times every second, and each crossing gives an arc a chance to extinguish. That is why AC switchgear works the way it does. A direct current circuit has no zero crossing. Once an arc is struck it is stable, and it keeps burning for as long as the source can feed it. A PV string behaves close to a constant current source, which is precisely the sort of source that sustains an arc rather than collapsing it.

An arc can ignite nearby combustible material. Opening a switch stops current only through the circuit it is designed to interrupt; other PV conductors can remain energised in daylight. The approved shutdown procedure must identify the equipment controlled and the parts that remain live.

Three kinds of arc, three different causes

• Series arc. A break in a current-carrying conductor. A badly crimped connector, a connector never fully clicked home, a fractured cell interconnect ribbon, a loose terminal in a combiner box. It is the most common and the easiest to miss, because the string keeps working at slightly reduced output while the joint heats.
• Parallel arc. Insulation failure between positive and negative. Rodent damage, cable chafed on the sharp edge of a purlin or tray, ordinary PVC building cable used outdoors and cooked by two seasons of sun.
• Earth arc. Conductor to earthed metal. Water into a junction box, damaged frame bonding, a self-drilling screw put through a cable during later roof works.

Use matched connector pairs with documented manufacturer compatibility, specified cable and correct assembly tools. Physical fit alone is not evidence of a safe electrical connection.

Arc fault detection, and what it can and cannot see

Some inverters include arc-fault detection. Confirm the exact model's supported protection, applicable certification, detection limits and required commissioning settings. A feature mentioned for a product family does not establish that it is fitted and active on the delivered unit.

Investigate recurring trips using the manufacturer's procedure rather than disabling protection. An inverter shutdown does not by itself establish that all array conductors are safe to touch.

Rapid shutdown, and what it actually achieves

Rapid-shutdown systems reduce voltage in specified parts of the PV installation when activated. Their scope and residual hazards depend on the design and equipment. US NEC requirements are not a substitute for checking Bangladesh requirements, the fire strategy and the authority having jurisdiction.

Have the electrical and fire-safety designers assess whether rapid shutdown or another hazard-control arrangement is required or appropriate. Include compatibility, access, maintainability and emergency-response needs in the specification; do not trade required protection against maintenance spending.

Specify isolation, switching, labelling and the operating sequence in the approved design. More isolators are not automatically safer: every device needs the correct rating, enclosure, location and maintenance arrangement.

Cable routing and fixings are a fire control

• Select cables for the applicable PV electrical, temperature and environmental duty.
• Support and protect cables clear of standing water and damaging edges using compatible fixings.
• Use manufacturer-approved UV/weather-resistant supports and inspect their condition.
• Minimise string loop area while maintaining the required fire, mechanical and electrical routing.
• Observe the specified bend radius and enclosure-entry requirements.

Getting a firefighter onto the roof

An array laid out purely for maximum installed capacity, with modules to the parapet and no walkways, is a roof the fire service cannot work on. The design decisions that decide this are made in an afternoon and cannot be undone later.

• A clear perimeter setback wide enough to walk the roof edge without standing on glass.
• Access routes through the array so the roof can be crossed and a ladder can be pitched.
• Smoke vents, roof hatches and existing firefighting penetrations left clear and marked on the layout drawing.
• A laminated single-line diagram and array layout at the main LT panel and at the plant entrance, showing every DC isolator and the array boundary.
• Labels in Bangla as well as English, including a warning that the array is live in daylight.
• An agreed instruction on what happens at night, when a shutdown device or an isolator behaves differently because the array is dark.

The step that actually helps is to walk the roof with the local Fire Service and Civil Defence station before the fire licence inspection rather than after it, and to leave them a copy of the layout drawing. A station that has already seen the roof will make better decisions at two in the morning than one reading a label for the first time.

Ten questions for a factory safety officer to ask

1. Show me the DC isolators. Where are they, are they labelled, and which one do we open first?
2. Are all the connectors on this roof from one manufacturer, and can you show me the crimp tool that was used?
3. What is the printed marking on the DC cable, and does it match what the specification asked for?
4. Do the inverters have arc fault detection, how many strings share a detector, and has it ever tripped?
5. When was the last thermal survey of connectors, combiner boxes and terminations, and can I see the images?
6. Where does the array earthing and equipotential bonding terminate, and when was continuity last measured?
7. Is there a walkway to the roof edge and a route across the array?
8. Where is the array layout drawing kept, and does the fire service have a copy?
9. What is the insulation resistance of each string today, and how does it compare with the commissioning value?
10. If a fire starts under the array at two in the afternoon, what is the written instruction for the shift electrician?

The last one is the real test. If the answer exists only in somebody's head, that is the gap, and it is the cheapest of all of these to close.

Sources and project checks

Consult the relevant edition and the documents for the exact equipment and site: IEC 62548-1: PV array design requirements; NREL: photovoltaic and energy storage operation and maintenance.

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