11.1 DC Isolation and Switching

Quick Answer: Handbook v1.3 Learning Outcome 4.2.4 requires isolation and switching at the PV string, at the sub-array on larger systems, and at the array. DC connectors may be used for off-load isolation only. Where connectors are used for isolation, an additional point of switching of the load must be provided, and that device may be within equipment such as an inverter. A DC switch-disconnector shall comply with BS EN 60947-1:2021, isolate all live conductors, and interrupt current in either direction. Off-load isolators must be marked so they are not disconnected under load. Switching-isolators must suit the maximum DC current and DC voltage the manufacturer specifies. This OpenExamPrep section is independent study material for those 2922-34 points; it is not a City & Guilds publication.

Why 4.2.4 is a design duty, not a roof habit

Learning Outcome 2 taught the sequence: shut the inverter down, isolate DC and AC, prove dead, lock off. Learning Outcome 4.2.4 asks whether the plant you designed actually contains devices that can do that job. Unit 301 places sixteen questions (53 percent) on design and installation. A method statement that says isolate DC is fiction if the only DC “isolator” on site is a pair of connectors still carrying operating current at noon.

The array is a generator that cannot be switched off at the laminate. Isolation is therefore downstream of the cells, at chosen points, with devices that match the duty: either they switch load (make and break current) or they only isolate after current has already stopped. Mixing those duties is how DC arcs start fires. UK fire-investigation work and the Fire Protection Association’s RC62 photovoltaic fire recommendations have repeatedly pointed at poorly selected or poorly installed DC switchgear and connectors. You do not need those titles as a 4.2.4 recall answer. You do need the handbook’s three isolation locations, the off-load connector rule, and a switch-disconnector that is actually a switch-disconnector.

Minimum isolation: string, sub-array, array

Handbook 4.2.4(a) is a minimum list, not a menu you can shorten because the roof is “only domestic.”

PV string. A string is a series group of modules. String isolation lets you open one generator circuit — a damaged connector, a failed module, a string fuse — without treating every other string under the same inverter as if it had gone dead. On a single-string domestic roof the string and the array are often the same circuit, so one correctly rated switch-disconnector at the inverter may serve both names. On a four-string combiner they are not the same circuit. Opening only the array switch still leaves live string inputs in the box. If you then pull a string fuse or unplug a string input, you have used a non-switched point under load unless current has already been stopped.

Sub-array (larger systems). A sub-array is a grouped portion of the array, typically after several strings have been combined and before the main array bus. Larger commercial and multi-roof plant uses sub-array isolation so that one roof face or one combiner group can be locked off while neighbouring groups stay generating. Handbook language flags this for larger systems. Do not invent a sub-array isolator on a 3 kW south roof that has one string. Do not omit group isolation on a farm roof with three combiners feeding one inverter if maintainers must work in one box while the others remain live.

Array. Array isolation separates the whole PV generator from the conversion equipment. That is the device you operate after inverter shutdown so the inverter DC terminals can be proved dead. The conductors on the array side of that open device remain live in daylight. Array isolation is not a claim that the roof is dead.

Missing any required point is a design defect. Extra points are allowed. Fewer than the minimum is not.

Non-switched isolation: connectors are not switches

Handbook 4.2.4(b) covers non-switched isolation points. Three facts belong together.

DC connectors may be used for off-load only. The plug and socket is a connection system. It is not a load-break switch-disconnector. Separating it while current flows draws a DC arc with no AC zero-crossing to help the plasma collapse. Off-load means another device has already stopped current. Cloud is not off-load. A blank inverter screen is not off-load. Night on a hybrid DC bus that a battery can still feed is not off-load.

Where connectors are used for isolation, an additional point of switching of the load must be provided. You may treat a mated pair as the isolation point for a string only if something else switched the load. That something else is a switch-disconnector, or a DC circuit-breaker that is suitable for isolation under BS 7671 — not an AC miniature circuit-breaker reused on a string, and not hope. Connectors plus a pair of pliers is not two duties.

The device switching the load may be within equipment such as an inverter. Many string inverters include an integral DC switch. If that switch is a true load-break isolating device for the circuit, it can be the additional switching point that makes connector isolation legitimate. If it is a plastic cover over DC plugs, it is not. The handbook allows the switching function to live inside listed equipment. It does not allow you to skip switching because the inverter has a logo.

DC switch-disconnector requirements

Handbook 4.2.4(c) is the product rule.

Compliance with BS EN 60947-1:2021. That is the number the 2922-34 handbook prints. BS EN 60947-1 is the general-rules standard for low-voltage switchgear and controlgear. For a 4.2.4 recall question, quote BS EN 60947-1:2021 as the named compliance standard for the DC switch-disconnector. IET selection articles also discuss the related switch-disconnector product standard and PV utilisation categories that distinguish off-load-only devices from devices that may make and break PV current. Use those categories to understand duty. Do not replace the handbook number with a different number when the paper is asking what 4.2.4 named.

Isolate all live conductors. On a DC pair that means both poles, not a single-pole handle that leaves the negative or the positive connected. A one-pole open still leaves a shock path and still leaves a return for current.

Interrupt current flow in either direction. Parallel strings can back-feed. A faulted string can see reverse current from its neighbours. Hybrid and battery plant can present current toward the array. A device imagined as breaking current only “from the roof toward the inverter” is the wrong picture. Bidirectional interruption is why the handbook says either direction.

Switching-isolators must suit maximum DC current and DC voltage as specified by the manufacturer. The cover rating is not a wish. You size from the array maximums taught in the ratings chapter — open-circuit voltage corrected for cold, short-circuit current corrected for high irradiance, combined current where strings share a bus — then you configure the device exactly as the manufacturer’s table and wiring diagram require. Bridging links that change two-pole series into a four-pole arrangement change the breaking rating. Using STC nameplate volts and amps instead of design maximums is a documented way to destroy a DC isolator when it is opened on load. Fine-stranded PV conductors often need ferrules if the terminals are not listed for Class 5 conductors. Torque to the stated value. Outdoor enclosures need glands, UV stability, and heat considered, not a south-wall box with an unprotected top entry.

Off-load isolators should be marked so they are not disconnected under load. If the device is only a disconnector, the marking is the control that stops someone treating the handle as a load-break switch. If the marking is missing, you cannot pretend the duty is obvious from the colour of the knob.

DC disconnectors incorporated within the inverter

Handbook 4.2.4(c)(iii) asks for considerations, not a slogan that inverter isolators are always enough or always banned.

Work through the product, not the brochure:

• Is the integral device a switch-disconnector (load-break and isolating) for this voltage and current, or a connector compartment?
• Does it isolate all live conductors and break both directions?
• Can it be locked off and labelled as the isolation point for the work?
• After it opens, what remains live? The array-side terminals and cables still see daylight generation.
• Does it give string isolation, or only array isolation at the inverter? Combiner work still needs the string-level provision.
• Do the manufacturer’s instructions still require an adjacent external DC switch-disconnector? Follow the instructions. The handbook does not override the product.
• Is the handle accessible without opening covers onto live parts, and reachable for emergency isolation (a garage wall versus a cramped loft hatch)?

An integral device that satisfies array isolation still leaves live DC between the modules and that switch. It does not make connectors into load-break tools.

On-load versus off-load

Scenario: unmating MC4s at noon to isolate

A two-string domestic array, clear south roof, 13:00 in June. The inverter is exporting. A trainee proposes pulling the string connectors at the eaves because that “isolates the inverter,” and the householder wants the job finished before school pick-up.

At noon the string is near operating current, voltage is high, and a DC arc at an unmated pair will not politely go out. The connectors are off-load only. Using them as the only isolation point, under load, is the defect 4.2.4(b) exists to stop.

The competent sequence uses the additional switching point: shut the inverter down using its internal control so load current falls; open a DC switch-disconnector that is rated to break this voltage and current (integral, adjacent, or both as the design requires); isolate AC as the dual-supply procedure requires; prove dead at the connectors; then unmate if the work needs the plug opened. If the only DC isolator on the quotation was the connector pair, the design never met 4.2.4. Finish the paperwork tomorrow rather than drawing an arc over a dry thatch, a plastic tray, or a timber eaves.