Fitting a Consumer Unit

The Board You Are Replacing Is Evidence

Open the old unit and read it for twenty minutes before quoting anything. Brown heat staining around one breaker's line terminal, three conductors crushed under a neutral screw designed for one, a bus stab that has arced itself grey, four brands of device in a board listed for one — each of those is telling you something about how the installation was extended and by whom. None of it appears on the plan the customer gives you.

Then check the directory against reality. Switch each way off and walk the house. A schedule that says KITCHEN and actually feeds the kitchen, the garage freezer and an outside socket is the version of the truth the next person will inherit, and you are the only one on the job with the outage window to establish it. Expect roughly half the labels to be wrong or partial in anything over twenty years old.

The expensive discoveries live in the existing installation, not in the board. Lighting circuits with no circuit protective conductor at the switch drops. Cable colours from two eras landed in the same terminal. A borrowed neutral running between two circuits that were never meant to share one, which stays invisible until the two circuits land on separate residual current devices and one of them refuses to stay in. Rubber-insulated tails that crumble when you flex them.

So price the discovery separately from the fitting. A changeover quoted as a fixed half-day, with an insulation resistance test done for the first time after the old board is on the floor, is how a straightforward job becomes a house with no power and an argument about who pays for a rewire. Test the outgoing circuits while the old unit is still energised and still someone else's, and put whatever fails in writing before you order the new one.

Five Parts Behind One Cover

A distribution board is a listed assembly, and the listing covers the whole thing together: enclosure, busbar, main device, outgoing devices and bars, tested as one unit. That matters more than it sounds. A breaker that fits the stab and carries the correct rating is not automatically a legal device in that board, because what was tested was the combination. Manufacturers publish the devices their assembly was evaluated with, and the label inside the door is the authority for what may land on that bus.

The busbar rating and the main device rating are separate figures and both belong on your check. A board with a 100 ampere bus and a 100 ampere main is fully committed on day one; the same enclosure with a larger bus leaves room for the service to grow without a second changeover. On three-phase boards the bus rating is also what governs how much you may load a single phase before the assembly, not the breaker, becomes the limit.

The bars are where the cheap failures live. One conductor per terminal unless the terminal is marked for more, no doubling of a neutral and an earth into a bar meant for one material, and every unused way blanked so the enclosure keeps the ingress rating it was listed with. A neutral bar with two conductors squeezed under a single screw will pass a visual inspection for years and then produce an intermittent fault that reads exactly like a loose connection somewhere out in the house.

Look at what the assembly is made of as well. Where the adopted rules require a non-combustible enclosure in a dwelling, or set conditions on siting a board under an escape route, that requirement is about the enclosure material and not about the devices inside it, and swapping a plastic unit for a steel one late in the job means a new backplate, new gland positions and possibly a new tail length.

Grouping Devices So One Fault Does Not Darken the House

How the outgoing circuits are grouped under residual current protection is the design decision an occupant actually experiences. A board split into two banks, each behind a single 30 milliampere device, means an earth fault on any one circuit takes half the house with it — and if the split was made by convenience instead of by function, that half can include all the lighting, the boiler and the freezer at once.

Individual residual current breakers with overload protection remove the problem at the cost of module width and unit price. Where the budget will not stretch to a full set, spend it where the consequences are worst: the freezer circuit, the heating controls, the alarm and any circuit feeding a medical device get their own protection, and the split, if there has to be one, keeps at least one lighting circuit on each side so a trip never leaves someone in the dark on a staircase.

Standing leakage is the reason a correctly wired board trips for no visible cause. Every switched-mode power supply in the house leaks a small current to earth through its input filter, and those currents add. Ten or fifteen milliamperes of accumulated standing leakage behind a 30 milliampere device leaves almost no margin, and the device then trips on a kettle or a shower that is doing nothing wrong. Measuring standing leakage per bank at commissioning turns that callback into a design note.

Arc-fault detection follows the same grouping logic and its own separate requirement. Which circuits need it, and whether combination devices are mandated, depends on the occupancy and the code edition the authority has actually adopted — a point worth confirming in writing, since arc-fault requirements have expanded in successive editions and adoption lags publication in many jurisdictions.

The Metalwork That Must Be Connected Before the Cover Goes On

Every main protective bonding conductor in the building lands on the earth bar of this board, and a changeover is the one moment when all of them are visible at once. Check each conductor is present, correctly sized for the supply arrangement you have, and continuous back to the clamp it claims to serve — the cross-sectional area demanded of a bonding conductor depends on the earthing arrangement of the supply and changes if that arrangement has been altered since the installation was built.

Continuity is the part that gets assumed. A gas bond clamped to a run of pipe that was later interrupted by a plastic section, or a water bond onto a stub of copper downstream of a plastic incoming main, reads as an existing bond and conducts nothing. Test it, do not look at it. The clamp being shiny proves only that someone fitted a clamp.

Where the supply arrangement itself needs establishing or the electrode resistance needs proving, that work belongs to its own sequence of measurements and its own guide; what belongs here is refusing to close the cover with a bonding conductor unaccounted for. A board energised over an unverified bond is an installation whose protective arrangement has never been demonstrated.

The Changeover, Hour by Hour

The supply side of the main switch is not yours to isolate. Depending on the jurisdiction and the metering arrangement, removing a service fuse or breaking a meter seal is the distributor's work, the supplier's work, or work you are authorised to do under a specific agreement — and the sequence, the notice period and the paperwork differ between all three. Establish which applies weeks ahead, not on the morning.

Everything testable gets tested while the old board is still standing. Insulation resistance circuit by circuit, continuity of protective conductors, and a note of anything that fails, signed off with the customer before you disconnect a single conductor. Once the old unit is on the floor, every pre-existing defect in the house becomes an argument about whether you caused it.

Work dead, prove dead, and prove the proving unit either side of the test. Lock off, label the lock, and keep the key on you. A second person switching something back on because the house went quiet is the accident this sequence exists to prevent.

1. Confirm who is permitted to isolate at the cutout and arrange it in advance, with the notice period the distributor requires.
2. Test every outgoing circuit at the old board and record the results with the customer present.
3. Isolate, prove dead at the incoming terminals, lock off and label.
4. Transfer circuits one at a time, identifying and labelling each conductor as it leaves the old board.
5. Land the main protective bonding conductors on the new earth bar and verify continuity to each service before the cover goes near the unit.
6. Torque every terminal to the marked value with a calibrated driver, working line, neutral and earth in a fixed order so none is skipped.
7. Complete the dead tests on the new board, then energise, then complete the live tests and the schedule.

Torque, Terminations and the Retighten Nobody Books

Terminal torque is a marked value on modern devices and a calibrated-driver job, not a wrist-feel job. Under-torque leaves a joint that heats, oxidises and heats further; over-torque crushes a conductor, damages the terminal thread and produces a joint that fails later for the opposite reason. Both failures look identical from outside the enclosure: a warm cover and an intermittent fault.

Preparation is half the joint. Strip to the length the terminal expects, so no bare conductor sits outside the clamp and no insulation is drawn inside it. Where a stranded conductor lands in a screw terminal, keep the strands together and land them square — a single strand folded outside the clamp both reduces the contact area and gives a live whisker somewhere inside a board that is about to be closed. Ferrules where the manufacturer calls for them, sized to the conductor, crimped with the matching tool.

Aluminium tails, where the service uses them, carry their own rules: a lug listed for the material, the antioxidant compound the manufacturer specifies, and torque to a marked value with a re-check policy. The joint that anneals slowly over a decade of thermal cycling is the reason those instructions exist.

Book the retighten. On a board carrying substantial sustained loads, a thermographic survey or a torque check at the first maintenance visit catches the joint that settled, and neither one takes an hour. Domestic work rarely gets that visit, which is an argument for getting the terminations right the first time rather than an argument that they do not matter.

Nothing Is Energised Until the Paperwork Says So

The test sequence has an order and the order is protective. Continuity of protective conductors and ring conductors, insulation resistance, and polarity are done dead, because each of them can reveal a condition that makes energising dangerous. Only then does the board go live for earth fault loop impedance, prospective fault current, residual current device operating times and functional checks of every switch and interlock.

Residual current devices get tested at their rated current and at multiples of it, with the trip times recorded against the limits in the adopted standard, and the integral test button pressed as a separate functional check. The button proves the mechanism; it proves nothing about the tripping current or the time, which is why both appear on the schedule.

The certificate is the deliverable. Depending on the jurisdiction that is an electrical installation certificate with a schedule of test results, or an inspection under a permit, and in some places both a notification to the building authority and one to the distributor. Which applies is a matter of local law, and none of it is optional because the work went well.

Finish with the schedule inside the door: every way identified by what it actually feeds, written from the switching you did during the changeover and not from the old directory. That schedule is the only part of this job the next person will read, and a schedule copied from a board you already proved was mislabelled has passed the error on with your signature under it.