Main protective bonding after a plastic pipe insert in an industrial unit
Your company is wiring a 200 m² light-industrial unit in Tsuen Wan. The supply is 380 V three-phase four-wire TN-C-S (PME) from the utility, rated 100 A, with the meter and main switch in an intake cupboard. The main earthing terminal is a brass bar in that cupboard, connected to the supplier's PME terminal by a 16 mm² earthing conductor. The incoming water pipe is 25 mm copper. Two weeks ago the plumbing contractor cut out a corroded section 1.2 m from where the pipe enters the floor slab and replaced it with a length of PEX, leaving copper on both sides. The gas pipe is steel and enters the unit at the rear wall, with the meter on the outside wall. A steel fire sprinkler main runs the length of the unit at high level, painted and bolted to the structure. The clerk of works says 'just bond everything to the water pipe where it comes in and we will be fine'. No main protective bonding has been installed yet.
1. Which services require main protective bonding here, where must each connection be made, and what conductor size would you use?
Read the worked answer
Identify the extraneous conductive parts first, then size the bonding.
Water: the incoming 25 mm copper is an extraneous conductive part and must be bonded, on the consumer's side of the stop valve, within 600 mm of the point where the pipe enters the building and before any branch. The clamp must be accessible, of the correct type for a metallic pipe, and labelled. That is exactly where the clerk of works is right - but his instruction stops there.
Gas: the steel gas pipe is an extraneous conductive part and must be bonded on the consumer's side of the meter outlet, within 600 mm of the meter, using an accessible clamp on the pipe - not on the meter body, and not on the utility's side.
Sprinkler main: it is metal and runs throughout the unit, but whether it is extraneous depends on measurement (see next task), not on appearance. Do not bond it by default; test it first.
Conductor size. For a TN-C-S supply with a neutral of 35 mm² or less, the main protective bonding conductor is 10 mm² copper; above 35 mm² it is half the neutral cross-section, with a minimum of 10 mm² and a maximum of 25 mm². Since the supply is 100 A, a 10 mm² bonding conductor to each bonded service is appropriate, run continuously to the MET without joins, and identified green/yellow throughout.
The plastic insert matters. Bonding the copper at the entry point does nothing for the copper downstream of the PEX, because the two are no longer electrically continuous. If the downstream pipework is to be bonded, it needs its own connection to the MET - and only after testing shows it is extraneous.
2. How would you decide, by test, whether the copper pipework downstream of the PEX section and the sprinkler main are extraneous conductive parts?
Read the worked answer
The test is a resistance measurement from the part to the main earthing terminal using an insulation resistance tester on 500 V DC.
- Isolate the part under test from any deliberate earth connection (disconnect any existing bonding clamp).
- Connect one lead to the MET, the other to a clean, bare area of the pipe or to a clamp on it.
- Press test and read the resistance. If the reading is at or above the accepted threshold of about 23 kΩ (0.023 MΩ), the part is not connected to earth through a path that can pass a dangerous current and is not an extraneous conductive part - it does not require main protective bonding. If the reading is below the threshold, it is extraneous and must be bonded with 10 mm².
The threshold comes from a simple risk calculation: 23 kΩ across 220 V passes about 10 mA, roughly the level at which a person can be held and cannot let go. Below that value, the part is capable of introducing earth potential into the installation and must be bonded.
Practical cautions:
- Test in dry conditions; a wet floor or damp pipe lagging will give a false low reading.
- Recognise parallel paths. A painted sprinkler main bolted to structural steel may read low because of the steelwork, not because of its own earth connection; investigate before bonding, and be aware that a low reading may also mean an unintended path that could carry fault current.
- Test both directions of the PEX insert: the upstream copper and the downstream copper separately, and record the results on the test sheet with the readings.
- Repeat after the plumbing is complete, because the PEX length and any new metal fittings will change the result.
3. The sprinkler main tests at 4 kΩ to the MET and the downstream water pipework tests at 60 kΩ. What do you do, and what do you tell the clerk of works?
Read the worked answer
Treat the two results very differently.
The sprinkler main at 4 kΩ is below the 23 kΩ threshold, so it is an extraneous conductive part. It must receive a 10 mm² main protective bonding conductor to the MET, connected with an accessible clamp on a clean, bare section of the pipe, labelled with the standard 'Safety Electrical Connection - Do Not Remove' notice, and recorded. A 4 kΩ path also tells you the main is in contact with something earthed - probably the structural steelwork - which is exactly the parallel path that makes a potential difference possible during a fault, and exactly why the bond is needed.
The downstream copper water pipework at 60 kΩ is above the threshold, so it is isolated from earth by the PEX insert and is not an extraneous conductive part. Bonding it would be pointless and could, in the wrong circumstances, import a potential that is not otherwise there. Do not bond it, and record the reading and the decision on the test sheet so the reasoning is visible to the next person.
What to tell the clerk of works:
- Bonding is decided by measurement and by the regulations, not by connecting every metal item to the nearest pipe.
- Each service gets its own 10 mm² conductor run back to the MET; bonding one service to another in a chain is not acceptable.
- The PEX insert means the water installation now has two electrically separate sections, and only the upstream section is bonded.
- The bonding connections must remain accessible and must not be buried or painted over - the decorator and the plumber both need to know that.