Kentec Syncro AS earth fault — step by step diagnosis

The Kentec Syncro AS at a glance

The Kentec Syncro AS is a popular addressable fire alarm panel in the UK mid-market. It works with Apollo, Hochiki ESP and Argus Vega loop devices (Kentec Man-1096, Syncro AS product manual) and is commonly found in commercial buildings, schools, and healthcare facilities. Earth fault is one of the faults signalled by its power supply monitoring: the manual's specification gives earth fault indication below 30 kΩ from the +28 V or 0 V line to earth.

Earth faults on Syncro AS systems are one of the more common call-outs, and they range from straightforward single-point insulation failures to intermittent issues that disappear when the test engineer arrives. This guide gives you a methodical approach to both.

Identifying an Earth Fault on the Syncro AS

When the Syncro AS detects an earth fault it shows EARTH FAULT, one of the fault conditions its power supply monitoring signals (Man-1096). It is a panel-wide indication: it tells you there is a leakage path from the panel's +28 V or 0 V line to earth somewhere, not which loop or circuit it is on. That shapes the whole diagnosis: the first job is finding which circuit carries the fault.

What this means: One of the loop conductors has a leakage path to earth — either through damaged cable insulation, a device with internal insulation failure, or a base with a short between a conductor and the earthed enclosure.

The Earth fault LED will be illuminated. The fault will have logged in the event history with a timestamp.

Initial Information Gathering

Before touching anything:

1. Check the event log — Has this fault appeared before? Repeated transient faults that restore themselves point to an intermittent insulation failure, often associated with temperature changes (cable expanding and touching metalwork) or moisture ingress. This changes your diagnostic approach.

2. List every circuit that leaves the panel — loops, sounder circuits, the auxiliary 24 V output, inputs and any network or repeater cabling. An earth fault can be on any of them, because the indication is not loop-specific.

3. Note the time and conditions — Earth faults sometimes appear only in wet weather (moisture in a junction box), after the heating comes on (thermal expansion), or after specific equipment is switched on (electrical interference coupling to the loop). Context matters.

Step-by-Step Diagnostic Procedure

Step 1: Find the circuit by disconnecting one at a time

With the Responsible Person notified and your safe system of work for taking detection out of service in place, disconnect the field circuits from the panel one at a time (loops, sounder circuits, auxiliary 24 V, inputs), waiting for the earth fault to clear or persist after each.

• Fault clears when a circuit is disconnected: the leakage is on that circuit's wiring or devices.
• Fault persists with every field circuit disconnected: suspect the panel or its own internal wiring, and contact Kentec technical support.

Reconnect each circuit before moving to the next so you don't leave more of the system out of service than you need to.

Step 2: Check the disconnected cable for earth fault

With the circuit disconnected from the panel, use a megohmmeter (insulation resistance tester). Caution: a 250 V or 500 V test applied to cable with loop devices still fitted can damage them. Remove the devices from their bases (or isolate the section) first, or confirm with the device manufacturer what test voltage their devices tolerate. Then:

• Measure the insulation resistance between each conductor and earth (the cable screen or a known good earth)
• Also measure between the two conductors

Healthy cable: Typically >100 MΩ. In practice, good wiring on a modern installation often measures into the GΩ range. Fault present: Resistance significantly below the healthy value — often a few kΩ to a few MΩ depending on severity.

If you get a healthy reading on the disconnected cable, reconnect one device at a time until the fault appears — it's likely a faulty device.

If the insulation resistance is low with no devices connected, the fault is in the cable itself.

Step 3: Section the circuit

Short-circuit isolators will not help here: they operate on a short between the loop conductors, not on leakage to earth. Instead, split the faulty circuit at accessible junction points (junction boxes or device bases roughly halfway along) and test each half, halving again until the faulty section is small enough to inspect. The site's as-fitted drawings save a lot of time at this step.

Step 4: Walk the cable route

Once you've narrowed the fault to a section, physically walk the cable route:

• Look for obvious damage: cable trays where cable has been bent, areas where the cable has been walked on, pinch points under conduit fittings
• Look for areas that could be damp: roof spaces, areas near roof lights, cable that runs close to pipework
• Look for areas where cable might be touching metalwork: trunking interiors, cable tray edges, where cable enters device bases

Step 5: Check device bases in the affected section

Remove each device in the affected section and inspect the base:

• Are both cable cores sitting clear of the earth terminal and the metal mounting plate?
• Is there any sign of moisture or contamination inside the base?
• Are the cable cores properly terminated — not nicked or frayed in a way that could allow contact?

A single core touching the earthed back plate of a surface mount base is a common cause of this fault. It often happens when cable is overtightened during installation.

Intermittent Earth Faults

Intermittent earth faults — ones that appear and then restore, or only appear at certain times — are harder to diagnose. Approaches:

• Thermal cycling: If the fault appears after the building heats up, look for cable that might be expanding and touching metalwork.
• Moisture: If the fault appears in wet weather, focus on external cable runs, roof spaces, and any areas with known dampness.
• Vibration: In industrial environments, vibration can cause intermittent contact. Look for cable in plant rooms or on rooftops.

For truly elusive intermittent faults, a data logger on the panel event log (or the Syncro AS's own event log retrieved via laptop software) can tell you the exact time pattern of fault occurrences, helping you correlate with environmental conditions.

Resolution

Cable damage: Repair or replace the damaged section. All joints must be in accessible junction boxes, properly terminated, and documented.

Device or base issue: Replace the device or base. If the base is fine but the cable core was touching the earth plate, re-terminate with the cores correctly routed.

Moisture ingress: Resolve the source of moisture as well as the immediate fault. A junction box with moisture in it will continue to cause problems if the root cause isn't addressed. Use appropriate IP-rated enclosures if the location requires it.

After resolving:

1. Recheck insulation resistance across the loop to confirm the fault is fully cleared
2. Reconnect to the panel and confirm the panel comes back to normal condition
3. Test affected devices to confirm they're reporting correctly
4. Document the fault, investigation, cause, and resolution in the system log book per BS 5839-1

Attending a Kentec Syncro fault on site? Create a free Incognito Fire & Security account to use the available diagnostic tools.