Testing & Teaching
Proving Unit
In short
A proving unit is a small box that gives a known test voltage. Its terminals: P1, P2.
If this is your home: this page is written for heating engineers, and for learners under the supervision of one. It explains how the wiring works; it is not training to do the work. In England and Wales, electrical work in a home must meet the Building Regulations (Part P), and some of it must be notified to building control or done by a registered electrician; Scotland and Northern Ireland have their own rules. For the wiring, call a registered electrician or a heating engineer competent in controls. Work inside a gas boiler is for a Gas Safe registered engineer only. If you smell gas, leave the building and call the gas emergency number: 0800 111 999 in Great Britain, 0800 002 001 in Northern Ireland.
What it is
A PROVING UNIT is a small box that gives a known test voltage. Put both of the two-pole tester's probes on its sockets: the tester lights, and is PROVED to work.
GS38, the Health and Safety Executive's (HSE's) guide to electrical test equipment, says: prove the tester before the dead test, make the test, then prove it again.
It gives nothing to the circuit: it is only for proving a tester.
Terminals
| Terminal | What it is for |
|---|---|
| P1 (Prove +) | One socket, for one probe. |
| P2 (Prove -) | The other socket, for the other probe. |
What the meter reads
Live tests. On site, any reading taken with the supply on is work on or near live parts. Regulation 14 of the Electricity at Work Regulations 1989 allows it only when working dead is unreasonable, as when a fault shows only while the controls work, and only with suitable precautions: test leads made to the HSE’s guidance note GS38, fingers behind the guards. Regulation 16 allows it only for a competent person, or a learner under the supervision of one. Wiring and repairs are always done dead. In the simulator, nothing is live.
Both probes on it: every light (LED) on the tester comes on, and so does its PROVED lamp.
Where it connects
Lessons that use it
- Why we work dead
Foundation · Safe isolation and the earth
- Prove dead
Foundation · Safe isolation and the earth
- The swap method
Intermediate · Swapping controls
- Safe isolation: prove dead before you start
Quick Jobs
Also in testing & teaching
Read the explanation
- Safe isolation in five steps: prove, confirm live, isolate, test every pair, prove again
Heating wiring is done dead. Prove the tester, confirm the supply is live, isolate and lock off, test every pair, prove the tester again. Why each step matters.
Practise it in the simulator.
Runs in the browser on a laptop or desktop. Cancel any time.