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 supports Apollo XP95 and Discovery protocols and is commonly found in commercial buildings, schools, and healthcare facilities. Like most addressable panels, it monitors loop wiring insulation continuously — any compromise will generate an earth fault condition.

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 will display a fault condition with a description referencing the loop and, on some firmware versions, an indication of which half of the loop the fault is on (the Syncro AS can identify which side of the loop a ground fault is on, because it monitors each half independently).

Panel display: Look for "Earth Fault L1" (or L2, L3 etc. for the relevant loop), or "Ground Fault" on earlier firmware. Some display formats will show "Loop 1 Earth" or similar.

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.

1. Note which loop — The Syncro AS will tell you which loop is affected. If it's showing a fault location (first half / second half of loop), note that too.

1. 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: Disconnect the loop at the panel

With the panel in fault investigation mode (notify the Responsible Person and follow your safe systems of work for temporarily disabling detection), disconnect the loop wiring from the panel terminals.

• Fault clears: The fault is in the field wiring or a device. The panel itself is fine.
• Fault persists: The fault is in the panel — unlikely but possible. Contact Kentec technical support.

Almost always the fault will clear when you disconnect the loop, confirming the issue is in the field.

Step 2: Check the disconnected cable for earth fault

With the loop disconnected from the panel, use a megohmmeter (insulation resistance tester) set to 250V or 500V DC:

• 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: Use isolator modules to section the loop

If your system has Apollo XP95 isolator modules (which it should, per BS 5839-1 and EN 54-17 requirements for cable lengths over the threshold), you can use them to section the loop and narrow down the fault location.

Access the panel's isolator module list (via Syncro AS menu > Devices > Isolators) to identify which isolators are fitted and their addresses. Tripping isolators manually allows you to disconnect sections of the loop and test each section in isolation.

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 5839-1:2025

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