4.3 Dual-Supply Isolation and Prove-Dead

4.3 Dual-Supply Isolation and Prove-Dead

PV systems are dual-supply equipment, and hybrid plant is often multi-supply. Energy can arrive from the array, from the grid, from another generator, and on hybrid jobs from a battery. Isolation that only opens the solar switch is not isolation. Handbook 2.2.1 walks through each location. Handbook 2.2.2 sets the procedure. Unit 302 will watch you do it. Independent study for 2922-34 requires you to know why each step exists, not only the order on a sticker inside the inverter door.

Safe isolation remains identify, isolate, lock off, prove the instrument, prove dead, prove the instrument again. Dual supply adds a counting rule: keep listing sources until none remain that can feed the point of work. Prove dead at all relevant points and at the times you are about to touch them. A test at 09:00 does not cover a return to the same terminals at 11:00 after cloud, after a householder, or after a colleague has operated a device.

Solar array

PV connectors must not be disconnected under load. They are connection devices, not load-break isolators. Separating them while current flows draws a DC arc that does not self-extinguish at a zero-crossing. The housing may be marked for voltage and current in the mated condition; that is not a licence to break load.

Modules are always considered live in conditions where they can generate. The inverter is off does not make the roof dead. Covering some modules does not make the remaining series group dead. Night on a hybrid system does not make the DC bus dead if a battery can still feed it.

Voltage can be present on either side of a PV connector where modules are in parallel and/or series. That sentence is the one candidates skip. In a series string, opening a connector still leaves Voc on the module side of each separated half of the string. In parallel, a neighbouring string can backfeed the combiner bus, so the inverter side of an opened connector can still be live. In mixed series-parallel, both effects stack. Never assume the far side of a plug is dead because you opened it. Prove dead on the conductors you will touch, not on a convenient nearby terminal that happens to be easy to reach from the scaffold.

Combiner box

A combiner gathers several strings onto a DC bus. Isolate and identify all sources: every string, any parallel jumper, any feed from another box, any DC-coupled battery on hybrid plant. Mitigate arc risk. Do not pull string fuses or unplug inputs under load. Shut generation down first. Use the manufacturer's isolation sequence. Wear eye protection. Stand aside when opening doors on boxes that may still be energised. Proving dead on one string input is not proving the bus. A fused string isolator that is not a load-break device is the same trap as a connector: no load before you operate it.

Combiner lids, gland plates, and metallic enclosures can be earthed. A live bus to the enclosure is a shock path. After isolation, still prove dead between poles and from each pole to earth before you put a tool on a terminal. If the box also contains surge protection, remember that some devices can present a path that surprises a tester; follow the product instructions rather than assuming a MOV looks like an open circuit.

DC isolation points

Ask two questions of every DC isolator or disconnection point. First: is this point suitable for switching or disconnection on load at this voltage and current? Load-break DC isolators are designed for that duty. Many connectors, fuse holders, and some enclosed switches are not. Second: if it is not suitable, ensure there is no load before anyone operates it. No load means the inverter has been shut down so it is not drawing current, and no other load remains — battery converter, DC optimiser chain still sinking current, or a second inverter on the same DC bus. Operating a non-load-break device under current is an arc incident waiting for a witness statement.

Suitable also means the device is rated for the DC voltage, the DC current, and DC use. An AC isolator in a DC string is not a suitable point. Polarity, enclosure IP rating, and the ability to lock the handle in the off position all matter when the isolator is the lock-off point.

Inverter

Isolate all AC and DC sources and outputs. The inverter is a bridge, not a barrier. DC can be present at the DC terminals from the array. AC can be present at the AC terminals from the grid or from another inverter. Internal capacitors can hold voltage after both sides are open. Manufacturer discharge times are part of the procedure, not optional waiting if you are in a hurry.

The AC output of the inverter may have a delay of up to five minutes after switching on. Do not read a dead AC test 30 seconds after switch-on as this inverter has no AC output. Conversely, after shutdown, do not assume the terminals are dead because the screen is blank. Prove dead. Be aware of residual charge and of delayed reconnection if someone else switches the unit back on. Anti-islanding and reconnection timers exist to protect the grid and the machine; they are not a substitute for isolation.

AC output from the inverter and other sources

Isolate all sources at other points: the generation-circuit isolator, the consumer-unit device, any battery inverter AC output, any backup or generator interlock, any other generating set that can feed the same tails. Dual supply means two directions. Multi-supply means you keep counting until there is no remaining source. A generation meter does not isolate. A display showing zero export does not isolate. A consumer-unit main switch that leaves the generation tails connected on the supply side of a downstream device does not isolate the inverter AC terminals.

Procedure from handbook 2.2.2

1. Shut down the inverter using its internal control so it stops converting and stops drawing DC load.
2. Isolate DC components and circuits — string isolators, combiner isolation, any DC-coupled battery isolator.
3. Isolate the input and output of the inverter (DC input and AC output, as fitted).
4. Isolate other AC sources that can feed the point of work.
5. Prove dead at all relevant points and at the times you are about to touch them: prove the tester on a proving unit or known live source, test the point, re-prove the tester. Repeat after a break, after a weather change, and after anyone operates a device.
6. Lock-off and label so the isolators cannot be closed while you are on the roof. Use your own padlock. A shared key is not lock-off. A caution notice should identify the work and who holds the key.

Prove-dead instruments must be rated for the DC voltage of the array and used with GS 38-compatible probes. A tester that only understands AC is not a PV prove-dead tool. Neon screwdrivers, non-contact wands intended for AC, and the inverter's own screen are not prove-dead instruments.

Scenario: hybrid with battery and grid

A hybrid inverter, rooftop array, lithium battery in the garage, grid connection, occupied house. At 21:00 the array is dark, so a naive installer isolates only the DC string isolator and starts work in the inverter. The battery can still feed the DC bus. The grid can still feed the AC terminals. The inverter may backfeed internally. Extra source means extra isolation: battery DC isolator, and any battery management isolator the manufacturer requires; grid and generation AC isolators; PV DC isolators; then prove dead on DC and AC at the inverter, at the battery terminals if that is the work location, and at any other point you will touch. Night does not make a hybrid system single-supply. A standby mode on a battery app is not isolation.

If the work is on the battery itself, treat stored energy as a separate source with its own shock, arc, and chemical hazards. Isolating the PV array does not isolate the battery. Isolating the grid does not isolate the battery. Prove dead after every source that can feed that enclosure has been isolated, and keep the householder away from the garage isolators while the padlocks are on.