Half-split fault-finding and why it works

It's 3am. The case packer won't start. The HMI says "safety circuit open" and nothing else. Behind it sits a chain of guard switches, two e-stops, a pull wire and a safety relay. Somewhere in that chain one contact is open.

The fitter on shift starts at the first guard switch and works along. Door one is fine. Door two is fine. Forty minutes later they're on switch nine, the line's still stood, and the shift manager has been over three times.

There's a faster way. It's called half-splitting, and it's the reason some people find a fault in ten minutes while others are still at it at 4am.

Start in the middle

The idea is simple. You have a circuit with a good end and a bad end. Signal goes in at one end and doesn't come out at the other. Instead of testing every point from the start, you test halfway along.

If the signal is there at the halfway point, the fault is in the second half. If it's missing, the fault is in the first half. Either way, you've just thrown away half the circuit with one test.

Then you do it again on the half that's left. And again.

Count the tests. Sixteen points in a chain, working from one end, could take sixteen tests. Half-split it and you're down to four. Thirty-two points, five tests. Every time the circuit doubles in size, you add one test. Working from one end, you double the tests.

Where to split

The middle of the circuit isn't always the middle of the drawing. Pick a split point you can get to easily, and one that's safe to test at. A terminal rail in the panel beats a switch halfway up a guard.

Some rules we'd use:

• Split where the circuit passes through a terminal strip, a plug or a junction box. You can test there without stripping anything.
• If one half is much easier to test than the other, split unevenly. A quick test that rules out a third of the circuit is still worth doing.
• Use what the machine tells you first. If the HMI or a safety relay shows which input dropped, start there. That's the machine half-splitting for you.
• If you know the history, use it. A guard that gets kicked by pallet trucks every week is worth checking early. Half-split is the method when you have no better clue.

Draw it before you test it

Half-split only works if you know which way the signal flows. On a breakdown, that means a drawing. If the panel drawings are in a drawer or on the server, get them. If they're not, sketch the bit you're working on. Supply, through each device, to the coil or the input.

HSE's guidance on electrical safe working practices makes the point that you need adequate information about the system before you work on it, and that it's unwise to rely on one source of information, such as a label (HSG85, paragraphs 36 and 37)*. Drawings get out of date. Someone added a door switch in 2019 and never marked it up. If what you measure doesn't match the drawing, believe the meter, and mark up the drawing when you're done.

Live testing is planned work

Here's the catch. Most half-split work on a control circuit means taking voltage readings with the panel on. That's live testing, and it needs to be thought through.

The Electricity at Work Regulations allow work on or near live conductors only when it's unreasonable for them to be dead, reasonable for the work to be done live, and suitable precautions are in place (regulation 14). HSE accepts that tracing a fault in a control system may not be possible with it dead (HSG85, paragraph 24). So fault-finding can pass the test. Poking about won't. The same guidance warns that commercial and time pressures will very rarely outweigh the risks of live work (HSG85, paragraph 31).

The same guidance has a line every maintenance manager should read twice. Electrical accidents often happen during fault-finding after a breakdown, when pressure to repair the equipment results in risks being taken. The answer HSE gives is to plan safe fault-finding procedures in advance, for use during breakdowns (HSG85, paragraph 38).

In practice, that means:

1. Test instruments with insulated probes and fused leads, in good condition (HSG85, paragraph 32). HSE's GS38 covers what good leads look like.
2. Shrouded terminals, and no reaching past exposed 400V to get to a 24V terminal.
3. Someone competent for that panel. A good mechanical fitter isn't automatically safe in a live drive cabinet. The regulations expect the right knowledge or experience, or supervision to match (regulation 16).
4. A clear line between testing and repair. Find the fault live, then isolate and prove dead before you touch it. Our guide to safe isolation covers that step.

Watch for the job changing under you. HSE warns that a testing job may turn into fault-finding (HSG85, paragraph 35). The same is true of fault-finding turning into a quick fix with the panel still on. That's where people get hurt.

When half-split lets you down

Half-split assumes one fault and a steady signal. Real breakdowns don't always play along.

Two faults at once will fool it. You find one open contact, fix it, and the circuit's still dead. Don't assume your first find was wrong. Test again from where you are.

Intermittent faults are worse. The signal's there when you test and gone when you walk away. Half-split still works, but you need the fault to show while you're watching. A meter with min/max, a PLC trend or a scope on the input does more than a quick look. We cover this in our guide to catching intermittent faults.

Loads that pull a supply down catch people too. A shorted sensor can drag a whole 24V rail low, so every input looks bad. If everything downstream is dead, check the supply before you split anything. Our guide to 24V DC control supply faults goes through it.

The cost of swapping parts

The alternative to a method is guessing. You've seen it. New sensor, still dead. New relay, still dead. New contactor. Two hours, three parts, and the fault was a loose wire on a terminal nobody tested.

Every part swapped at random costs the part, the time and the restart. The worst cost is the hour the line sits while someone hopes. If you've worked out what an hour of downtime costs you, you'll know a structured engineer pays for themselves on the first night.

Here's our view. Half-split is a skill you can teach. Every fitter and sparky on your team should be able to explain it, and show you on a drawing. If someone on shift can't, that tells you who needs training and who you'd want on call for electrical faults.

It also tells you when to stop. If half-split has run out of road and nobody on shift knows the kit, that's the moment to call for help. Our 30-minute rule sets out when.

What to do on Monday

1. Check the panel drawings for your three worst machines are on site, current and somewhere the night shift can find them.
2. Write a short fault-finding procedure for live testing on those panels: who can do it, which meter, and when to isolate.
3. Ask each fitter and sparky to talk you through half-splitting a safety circuit on a drawing. Note who can't.
4. Check every test lead on site against GS38. Bin the ones that fail.

* Paragraph numbers correct as of 26 August 2026 and refer to HSG85, third edition (2013). Check HSE's HSG85 page for the latest.