Foundation · Unit 7
Testing
In short
Polarity at the spur, the wiring centre and a load; earth continuity to every load; tracing a demand probe-to-neutral on the S-Plan and the Y-Plan; the last-230-V, first-0-V rule on a broken system; commissioning against the truth table. Every reading taken with the meter.
Comes after The Draw Warm Method: Y-Plan.
In this unit
Polarity: the spur, the centre, a load
Live (L) to neutral (N) reads 230 V at the source. Live to earth (E) reads 230 V and neutral to earth 0 V, at the spur, the wiring centre and the boiler: seven readings.
How it opensIn this lesson you test POLARITY at three places: the fused spur, the wiring centre and a load.
Earth continuity to every load
The system isolated and proved dead first. Then the meter on continuity, from the spur's earth to the boiler, the pump and both valves: four beeps, the probes lifted each time.
Tracing a demand: the S-Plan
The meter's black probe on a neutral, its red probe on each point in turn: block 4, block 5, the valve's brown and orange, block 10, the boiler's switched live (SL).
Tracing a demand: the Y-Plan
Heating only on a Y-Plan: hot water off puts the grey on block 7, white and grey take the 3-port valve to A, and its orange on block 8 fires the boiler.
The last 230 V, the first 0 V
A heating demand that stops somewhere: find the last point with 230 V and the first with 0 V. The fault is between them.
Commissioning against the truth table
Heating only, hot water only, then both channels on with both thermostats (stats) satisfied: the interlock, proven with the meter on the oranges' block.
You will learn to
- Test polarity at the source, the wiring centre and a load, and read a reversed supply
- Test earth continuity dead from the supply to every load
- Trace a demand probe-to-neutral in the order of the numbers, on an S-Plan and a Y-Plan
- Find a break with the last-230-V, first-0-V rule
- Commission a system against its truth table with the meter on the oranges
Checks and credit
- A timed check at the end of the unit, its questions drawn at random. The score goes on your record.
- The unit earns the credential Testing and Commissioning (simulator).
- Also a short course on its own, Testing and Commissioning, with credit both ways.
Where it connects
The parts it uses
Foundation
- The Draw Warm Method: Y-Plan
Unit 6, before this one
- Jobs
Unit 8, next
Back to the Programme.
Read the explanation
- Last 230 V, first 0 V: the rule that finds a fault with three readings
Heating fault finding with a meter: follow the demand point to point, probe to a neutral, and the fault lies between the last 230 V and the first 0 V.
- What a truth table proves when you commission a heating system
Commissioning proves every state of the controls before handover. The truth table’s rows, the boiler interlock row by row, and the dead tests that come first.
Practise it in the simulator.
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