Series Circuit Earth‑Fault Detector

A Series Circuit Earth‑Fault Detector (also known as a ground‑fault detector or leakage monitor) is a specialised protection device that monitors the insulation integrity of an AGL series lighting circuit. It continuously measures the current flowing from the circuit conductors to earth (ground) and provides an alarm or trips the system when a fault current exceeds a preset threshold.

In AGL series circuits, the isolating transformers are designed with primary and secondary windings that are electrically isolated from each other and from earth. However, cable insulation can degrade over time due to ageing, mechanical damage, moisture ingress, or chemical attack. When insulation fails, a leakage path to earth is created, which can cause nuisance tripping, create a shock hazard, or lead to a complete circuit failure. The earth‑fault detector provides an early warning of such degradation, allowing maintenance to be scheduled before a full failure occurs.

The earth-fault detector exists to catch a specific and dangerous failure mode that the isolating transformers themselves are designed to prevent from spreading, but can't prevent from starting: insulation on a cable or fixture slowly degrading from age, moisture, or mechanical damage until current begins leaking to ground. Left undetected, that leakage path can create a shock hazard for anyone working near the circuit and can eventually progress to a full insulation failure that takes the fixture, or worse, part of the loop, out of service.

By continuously monitoring for this leakage and alarming well before it becomes a complete failure, the detector turns an otherwise invisible, slow-developing hazard into an actionable maintenance item — giving technicians the chance to locate and repair degrading insulation on their own schedule, rather than discovering the problem only when it causes an unplanned outage or, worse, an actual shock incident.

Series Circuit Earth‑Fault Detectors are installed in airfield electrical rooms, equipment shelters, and AGL power centres. They are used in all types of AGL series lighting installations where insulation monitoring is required, including:

• Runway Lighting Systems: Monitoring edge, centreline, threshold, and touchdown zone circuits for insulation degradation.
• Taxiway Lighting Systems: Protecting taxiway edge, centreline, and stop bar circuits from earth faults.
• Approach Lighting Systems: Ensuring the integrity of approach lighting circuits (ALSF‑1, ALSF‑2, MALSR, etc.).
• Precision Approach Runways (CAT I, II, III): Where high reliability is mandated, and early fault detection is essential to prevent circuit failures during low‑visibility operations.
• Integration with ALCMS: Providing real‑time leakage current data and alarms to the central control system for remote monitoring and maintenance planning.
• Preventive Maintenance Programs: Enabling condition‑based maintenance by tracking insulation resistance trends over time.

The detector is installed in the equipment shelter alongside the CCR and series circuit it protects, wired to sense leakage current across the full loop rather than at any single point, since an earth fault can develop anywhere along the buried cable run or at any fixture connected to it. Sensing connections are typically made at the CCR output or via a dedicated current transformer around the full set of circuit conductors, positioned so both the outgoing and return paths are measured together and any imbalance between them is detected as leakage to earth. Alarm outputs are wired back to the ALCMS alongside other AGL monitoring cabling, clearly labelled to the specific circuit being protected in equipment rooms serving multiple regulators. Because the detector's usefulness depends on catching insulation degradation early, it's commissioned with a baseline leakage reading for the healthy circuit, giving a reference against which a developing fault can be measured as it progresses.

The earth‑fault detector operates by injecting a small, safe test signal (typically a low‑frequency AC voltage or DC pulse) between the series circuit and earth, or by using a current transformer to monitor the imbalance between the circuit conductors. The most common methods include:

• Residual Current Monitoring: A differential current transformer (CT) is placed around both conductors of the series circuit. Under normal conditions, the algebraic sum of currents in the two conductors is zero. When a fault to earth occurs, a portion of the current returns via earth, causing an imbalance that is detected by the CT.
• Insulation Monitoring (Active Method): A test signal is injected between the circuit and earth, and the resulting leakage current is measured. This method can detect insulation degradation even when the circuit is de‑energised, and it provides a quantitative measurement of insulation resistance (typically in megohms).
• Voltage Monitoring: Measures the voltage between the circuit conductors and earth. A significant shift indicates a change in the balance of the circuit and can be used to infer the presence of an earth fault.

When a fault is detected, the detector can either raise an alarm (via dry contacts, LEDs, or communication) or, in some systems, trip the CCR to de‑energise the circuit. The threshold for alarm or trip is adjustable, typically ranging from a few milliamperes to several hundred milliamperes, depending on the sensitivity required.

The detector sits inline with the CCR's own fault monitoring rather than as a separate control layer — it adds a fault class (earth leakage) that the CCR's current/voltage regulation loop cannot see on its own, since a balanced leakage path doesn't necessarily register as an open or short circuit. Where wired to trip the CCR directly, the detector's alarm threshold effectively becomes another input into the loop's overall protection logic, sitting alongside the CCR's regulation function rather than overriding it. Where wired only to raise an alarm via dry contact or comms to ALCMS, the circuit stays energised and the decision to de-energise is left to an operator or a higher-level automated response, rather than being made locally by the detector.

The detector is built around a sensitive current transformer, typically a zero-sequence or core-balance CT, clamped around all the series-circuit conductors together so it responds only to the small imbalance current that flows to earth through an insulation fault rather than to normal load current. This sensor feeds a signal-conditioning and threshold-comparison circuit housed in a panel- or rack-mount case, isolated from the series circuit's operating voltage by the CT itself and by internal isolation barriers on the sensing input. An adjustable threshold and time-delay setting, set via front-panel controls or dip switches, is provided so nuisance trips from normal leakage (such as damp cable insulation within acceptable limits) can be distinguished from a genuine developing fault. Output is via an alarm relay and, on networked units, a communications port, with rear terminals bringing in the CT leads, auxiliary power, and alarm/communications wiring into clearly separated terminal groups.

Airport Series Circuit Earth Fault

An airport series circuit earth fault is a leakage path from the circuit conductors to earth. It usually starts as insulation degradation from ageing, mechanical damage, moisture ingress or chemical attack. Isolating transformers keep the primary and secondary windings isolated from each other and from earth, so a healthy series circuit has no deliberate path to earth.

The earth-fault detector monitors for this leakage and raises an alarm or trips when it exceeds the set level, giving early warning before the insulation breakdown becomes a shock hazard or a complete circuit failure.

An alarm shows that a circuit has a problem but not where. Where the installation allows sections of the loop to be isolated, sectionalising against the leakage reading narrows the location faster than inspecting the whole loop. Treat every earth-fault alarm as a real fault until checked, and do not disable or ignore it to keep a circuit in service. Check the primary cable and the isolating transformers as the usual places for the leakage.