12.1 Insulation Monitoring, RCM and Earth-Fault Alarms

Why DC earth-fault monitoring is on the 2922 knowledge test

A small grid-tied photovoltaic (PV) array is a DC source that keeps generating whenever there is daylight. On a typical UK string the DC side is unearthed (an IT arrangement in BS 7671 language): there is no intentional connection from either pole to the protective earth, so a first earth fault often will not operate an overcurrent device. There is no metallic short until a second fault appears. Leakage can instead pass through damp timber, foil-backed loft insulation, a metal roof sheet, a wet connector, or a nicked cable under the modules. That path can heat for hours. Direct current has no natural zero crossing, so a carbonised track can hold an arc that AC at 50 Hz would have a better chance of extinguishing.

That is why City & Guilds 2922-34, qualification handbook version 1.3 (August 2025), learning outcome 4.2.6, treats insulation-resistance monitoring devices, residual current monitoring (RCM) systems, and earth-fault alarms as design selections, not as optional extras the inverter salesman mentions in passing. Independent OpenExamPrep teaching for this unit uses those handbook rules and the same Section 712 logic you already apply as an electrician. OpenExamPrep is not a City & Guilds partner and does not speak for the awarding body.

If you leave a generating DC earth fault undetected, you have not installed a quiet system. You have installed a slow ignition source in a roof void.

Insulation-resistance monitoring devices

An insulation monitoring device (IMD) watches the resistance between the live DC conductors and earth. On a dry, undamaged Class II string that figure is high — commonly in the megohm range. Water in an MC4, a rodent-chewed sheath, UV-cracked insulation, or a crushed cable behind a rail pulls the resistance down. The IMD compares the reading with a manufacturer threshold and raises a fault when insulation has collapsed.

Most modern string inverters run this as an Riso or isolation measurement at wake-up, and some continue to supervise insulation in service. The handbook also allows a separate IMD in DC switchgear or a combiner, which you will see on older transformer inverters or on specifications that demand a dedicated monitor.

Insulation monitoring is about the state of the insulation, not about how many milliamps happen to be flowing at this instant. A wet connector on a dull January morning can fail the megohm check before residual current is dramatic. Do not tell the examiner that “the array is Class II so we do not need monitoring.” Class II construction reduces the chance of a shock from exposed metal; it does not make water ingress or rodent damage impossible, and it does not remove 4.2.6.

Residual current monitoring systems

An RCM system looks at current balance. In a healthy two-wire DC string, current in the positive conductor equals current in the negative conductor. If some current leaves the circuit to earth — through a person, a damp rafter, a failed gland, or a module frame fault — the two currents no longer match. The RCM detects that residual and the inverter typically stops converting and disconnects from the AC side under its product standard.

Keep the names straight for the e-volve test:

• IMD / insulation resistance — resistance from DC live parts to earth.
• RCM — imbalance between positive and negative (or between the DC circuit and earth as a residual).
• RCD — an AC residual current device on the consumer-unit side. That is a later handbook topic (AC isolation and RCD selection). A 30 mA Type A on a lighting circuit is not the array earth-fault device.

Transformerless inverters almost always include residual-current monitoring because the DC bus sits on the same hardware that ties to the AC grid. That integral function counts as “protection within inverters” — if the alarm rules below are also met.

Location: inside the inverter or a separate device

Handbook 4.2.6(c) allows both locations:

• Protection within inverters — the usual domestic case. The string inverter measures insulation at start-up, monitors residual current in service, and shows a ground-fault or isolation-fault indication.
• Protection from separate devices — used where the inverter does not provide the function, where a DC assembly includes its own IMD or RCM, or where a client specification wants a dedicated monitor feeding a remote sounder.

Either location is acceptable only if the earth-fault alarm behaviour is met. An inverter that measures Riso but only blinks a five-millimetre LED on a loft boarding sheet has not finished 4.2.6. The monitoring electronics can live in the inverter; the alarm the operator will actually notice may still need a separate sounder, beacon, or documented remote indication.

The three earth-fault alarm rules

Memorise handbook 4.2.6(d) in the order the handbook writes it. Earth fault alarms must:

1. Not rely solely on a flashing light on an inverter to indicate an earth fault.
2. Continue to operate until the system is switched off or the fault is corrected.
3. Be accompanied by instructions to the system operator on the need to immediately investigate the fault.

Rule 1 exists because a flashing inverter LED is an installer convenience, not an occupant alarm. Rule 2 exists so a self-resetting blink that clears when cloud covers the array, or when the inverter sleeps at night, cannot hide a still-wet connector. Rule 3 exists because the person who lives with the system is usually not an electrician. Handover must tell them that a persistent earth-fault indication is an investigate-now event, not a “see how it looks next weekend” event.

A timed auto-reset, a three-flash-and-stop pattern, or an indication that vanishes when generation falls to zero all fail rule 2. A PDF manual left in the loft hatch, never explained, fails rule 3.

Why a garage inverter LED is useless at 02:00

Picture a 3.6 kW array on a semi-detached house. The inverter is on the garage back wall because that kept the DC cable short and the garage was cooler than the loft. At 02:00 an insulation fault develops: condensation in a poorly crimped MC4, or a slow drip onto a nicked cable under the array. The inverter LED flashes. Nobody is in the garage. The occupants are asleep in the dwelling. The LED does not wake them, does not page a phone they were shown how to use, and does not sound in a hallway they walk through to make tea.

At dawn the array starts generating into the fault. Leakage can persist for the whole generating day. Roof voids, garage ceilings, and foil insulation are exactly the fuels you do not want on that path. An examiner who asks why the flashing light is not enough wants this occupancy argument: the people at risk cannot see the lamp.

A suitable arrangement uses something the operator will actually notice — a dedicated sounder or beacon in a regularly occupied space, or inverter monitoring that produces a persistent occupied-space or documented remote indication — and written instructions to investigate immediately. The alarm must still be there after you walk away.

Scenario: landlord property, inverter in loft, LED-only alarm

A buy-to-let mid-terrace has a single string feeding an inverter mounted on loft boarding. The installer leaves the inverter’s red earth-fault LED as the only indication. The tenant never enters the loft except to put a suitcase down in January. The landlord lives in another town and was emailed a manufacturer PDF they did not open.

This is an exam-fail arrangement. It fails all three handbook tests: the indication is solely a light on the inverter; if that light drops out when the inverter shuts down at night the operator has no continuing warning; and there are no effective instructions to a system operator who will actually act. The system operator on a rented house is the landlord or a named managing agent, not an assumed “whoever has the loft hatch.” A competent design puts a persistent alarm where that operator (or a resident they have briefed) will see or hear it, and files handover that says: isolate AC and DC if it is safe to do so, keep people away from the array, and call a competent person immediately.

On the knowledge test, if the stem mentions a flashing inverter lamp and nothing else, treat it as a fail unless the item also describes a persistent occupied-space alarm and operator instructions.