Prove a part with two readings before you condemn it

A motor needs a live and a neutral

Current flows in a loop. It goes out on the live, through the part, and back on the neutral. A valve’s motor, a pump, a receiver’s relay: none of them works on a live alone.

So a live on a part’s terminal is not enough to blame the part. It proves the call has arrived. It does not prove the part has what it needs to answer.

That gap between “the live is there” and “the part can use it” is where good parts get thrown away.

Brown to earth reads 230 V and the valve does not move

Take a 2-port zone valve on an S-Plan. The heating is on, the room thermostat calls, and the valve stays shut. You put the meter on the valve’s brown core and earth. It reads 230 volts (230 V).

The tempting answer is a dead motor, and a new head. Not yet.

That reading proves the live reaches the brown. So the supply, the programmer, the thermostat and the wires up to the valve all work. It says nothing at all about the blue, the motor’s neutral.

These readings are live tests. Regulation 14 of the Electricity at Work Regulations 1989 allows live work only where it is unreasonable to work dead, it is reasonable to work live, and suitable precautions are taken: here, because a demand exists only while the controls work. Regulation 16 of the same Regulations allows them only for a competent person, or a learner under the supervision of one. Use leads made to the HSE’s guidance note GS38, with your fingers behind the guards.

Brown to its own neutral

Now move the black probe from earth to the valve’s own blue, at the valve. Not to a neutral at the wiring centre: that would skip the very wire you need to test.

Two answers are possible.

• About 230 V across brown and blue, and the valve does not move. The motor has both halves of its supply and does nothing. Now the motor is the suspect, with evidence. Isolated, proved dead and disconnected, its motor winding should show a resistance on the meter, close to the maker’s figure where one is given. A winding that reads open, no path at all, has failed.
• 0 V across brown and blue, while brown still reads 230 V to earth. The neutral is missing: a loose screw, a broken core, a neutral never landed. A new head would not move either. This is the case the second reading exists for.

With a broken neutral, the neutral screw at the part can even read 230 V to earth, fed through the motor’s own winding. The part has a live and still cannot work. That is why 0 V across a part never proves it dead.

The same two readings work on a pump: its live to its own neutral, inside its terminal box. Full supply and not running points at the pump. No supply sends you back along its feed.

A fuse blows for a reason

A blown fuse is a symptom, not the fault. Something drew more current than the circuit was meant to carry: a short circuit, a motor that cannot turn, water where it should not be.

Find that reason first, with the circuit isolated and proved dead. Then fit a fuse of the rating the maker of the boiler or the controls asks for. Never fit a bigger fuse. The fuse protects the cable and the controls; a bigger one lets a fault heat the cable instead of breaking the circuit.

To test a fuse, take it out and test it dead for continuity. A fuse is never proved good by looking at it.

Why swapping parts is the expensive way

A part changed on a hunch costs the customer for the part and for the time. If the fault was a wire, it is still there when you leave, and the call comes back.

Two readings take a minute. They tell you which part has failed, or that none has. Follow the demand first, and the fault lies between the last point with 230 V and the first without. See last 230 V, first 0 V.

Replace, then prove the repair

When the part is proved faulty:

1. Isolate, lock off and prove dead, the switched lives included. See safe isolation in five steps.
2. Fit the new part like for like, and wire it core by core from its own map, not from the old one’s colours.
3. Prove earth continuity to it, and close every cover, before the power goes back on.
4. Restore the supply and set the demand that failed. Watch the part do its job: the valve opens, the end switch closes, the boiler fires.

The lesson says: “Never leave a job until you have seen the repair work.” Then leave the system as you found it, apart from the fault you mended.