Three-Phase Bakery with PV, Generator and a Borrowed Neutral
A bakery in Limassol has a 400/230 V TN-C-S supply with a 100 A main switch. The main board feeds a 40 A three-phase oven isolator, a 20 kW standby diesel generator with an automatic changeover, and a 6 kWp PV inverter connected on the load side of the main switch. The electrician is to replace the oven isolator, which is burnt. The main switch is switched off; a voltage indicator shows 230 V between line and neutral at the oven terminals. The site supervisor says the generator 'only runs during power cuts' and that the PV 'shuts down when the grid is off'. A single-phase shop-front sub-board is fed from one outgoing way, but its neutral is borrowed from a different way in the same board.
1. Explain why 230 V is present at the oven terminals with the main switch off, and set out a safe isolation procedure for this installation.
Read the worked answer
230 V line-neutral with the main switch open is a real backfeed, not a ghost voltage. The PV inverter is connected on the load side of the main switch, so with the main switch off the inverter can still energise the busbar until its dedicated AC isolator is opened and its anti-islanding has operated. The standby generator is a second source; an automatic changeover may switch only three poles, leaving the neutral connected. The borrowed neutral also holds the neutral at a potential above earth. The main switch is therefore not the point of isolation for all sources.
Procedure:
- Identify every source: mains, PV, generator, any UPS, and the shop-front sub-board.
- Isolate the PV at its dedicated AC isolator and lock it off.
- Isolate and lock off the generator; confirm whether the changeover is three-pole or four-pole and whether the neutral is switched.
- Isolate the main supply and lock off.
- Apply warning notices and personal locks at each point.
- Prove the voltage indicator on a proving unit, then test line-line, line-neutral, line-earth and neutral-earth at the oven and at the board.
- Prove the indicator again before working.
Only when every source is under control and all conductors test dead is the permit valid.
2. How do you deal with the neutral and the multiple sources, and what would you do if one source cannot be isolated?
Read the worked answer
Treat the neutral as a live conductor. With a three-pole changeover the generator neutral remains tied to the installation neutral, so a running generator can raise the neutral above earth even when all line poles are open. Options are to use a four-pole changeover with a switched neutral, isolate the generator neutral, or arrange a full generator shutdown. If the neutral cannot be isolated, do not work.
The borrowed neutral means the shop-front sub-board and the circuit using that neutral must both be isolated. Otherwise the neutral is a parallel path and can be live at the point you are working. Lock off both boards, label them, and prove the neutral dead as well as the lines.
If any source cannot be locked off, stop and escalate to the site supervisor and, if necessary, the DNO or the person in control of the generator/PV. A permit to work is only valid when every source is under control. Record the borrowed neutral as a defect - it is a shock risk to the next person and can cause overheating from parallel neutral currents.
3. What checks and records are needed before re-energising, and what must the client be told about the separate sources?
Read the worked answer
Before re-energising:
- visual check of the new isolator: rating, terminal torque, phase sequence and motor rotation for the oven, IP rating, cable glands and earthing;
- insulation resistance, continuity of the CPC, Zs and RCD tests as applicable;
- confirm the PV and generator lockouts are removed only by the person who applied them;
- check the changeover and anti-islanding settings, and replace all covers and labels;
- function-test the oven and the changeover, then cancel the permit.
Records: a minor works certificate or EIC depending on the extent, an updated schematic showing the PV and generator, RCD test results, and the borrowed neutral as an observation. Tell the client in writing that the PV and generator are separate sources and must be isolated before any work, and that the borrowed neutral must be corrected. The supervisor's assurance that the generator 'only runs during power cuts' is not an isolation procedure.