The short answer
The main switch on a consumer unit is a double-pole isolator that disconnects both the live and neutral conductors simultaneously, cutting power to every circuit fed from that board. On a typical domestic consumer unit it is rated at 100 A — chosen to handle the maximum demand likely from a standard domestic installation — and sits at the left-hand end of the board. Turning it off makes the board circuits dead and safe to work on, but does not isolate the incoming meter tails, which remain live because they are on the supply side of the meter. Some consumer units use a main switch that is also an RCD (called an RCD incomer or main switch RCD), giving the whole installation residual-current protection from a single device at the top of the board. Under BS 7671 18th Edition, every consumer unit must have a main switch that provides a means of isolation for the whole installation.
The main switch is the largest switch on your consumer unit — the one that cuts all power to the board when you need to work on a circuit. Understanding what it does, and what it does not isolate, is important for safe working and for understanding how your board is protected.
Main switch key facts
- TypeDouble-pole isolator (disconnects live + neutral)
- Typical domestic rating100 A
- What it isolatesAll circuits fed from the board
- What it does NOT isolateIncoming meter tails (remain live)
- May also beRCD incomer (combined main switch + RCD)
What 'double-pole' means and why it matters
A double-pole switch disconnects both the live conductor and the neutral conductor at the same time. A single-pole switch disconnects only the live. For an isolator at the top of a consumer unit, double-pole is essential: in a correctly wired installation the neutral is at earth potential, but if there is a neutral-to-earth fault elsewhere on the network, the neutral can become live. A double-pole main switch ensures you are properly isolated from both conductors, not just the expected live.
This is why BS 7671 requires the main switch to be double-pole. It also means that when the main switch is off, both conductors at all downstream terminals are dead, which is the safe condition for working inside the board.
The 100 A rating — what it means for your installation
The 100 A rating does not mean your home can draw 100 A continuously. It means the switch is capable of carrying that current and interrupting it safely. In practice, the maximum demand of a typical UK domestic installation is calculated by the designer and is usually well below 100 A — a standard connection to the grid via a domestic cut-out fuse is commonly rated at 60 A or 100 A depending on the tariff and property.
A 100 A main switch is used as a standard because it provides adequate margin for virtually any domestic load, even with an EV charger or electric shower added. If an installation is designed to draw more than the standard cut-out fuse allows, the supply capacity would need to be agreed with the distribution network operator (DNO) — that is a supply-side matter, not a consumer unit sizing matter.
Main switch versus RCD incomer
On some consumer units the main switch is a plain isolator — it provides isolation only, with no residual-current detection. On others, the main switch is a combined main switch RCD (or RCD incomer): it both isolates and monitors the whole installation for earth-leakage faults. If an earth fault occurs anywhere in the installation, the RCD incomer trips the whole board.
The RCD incomer arrangement was common in older consumer units and provides whole-installation protection from a single device. Its drawback is that any earth fault trips the entire board, leaving the whole property without power until the fault is found. This is why the split-load arrangement (two RCDs each covering half the board) became standard with the 17th Edition of BS 7671, and why full RCBO boards are the preferred approach today — both give finer isolation, so a fault on one circuit does not affect the rest.
| Main switch type | Provides isolation | Provides RCD protection | Effect of earth fault |
|---|---|---|---|
| Plain isolator | Yes | No | Depends on downstream RCDs |
| RCD incomer (main switch RCD) | Yes | Yes — whole board | Whole board trips |
| Isolator + downstream dual RCDs | Yes (isolator) | Yes — half-board sections | Half the board trips |
| Isolator + RCBOs per circuit | Yes (isolator) | Yes — per circuit | Single circuit trips |
Main switch configurations and their effect on fault isolation. Source: BS 7671:2018+A2:2022.
Frequently asked questions
Will turning off the main switch cut all power in my home?
It will cut power to all circuits that are fed from that consumer unit. If your home has more than one consumer unit (for example, a separate board for a garage or an annexe), the other boards will remain live. The incoming meter tails will also remain live.
Can I turn off the main switch to change a light switch or socket?
Turning off the main switch de-energises the circuits in that consumer unit, which makes it safer than working live. However, consumer unit replacement and modification work is notifiable Part P work requiring a registered electrician. Changing a like-for-like switch or socket is lower-risk but still requires care; if in doubt, consult a registered electrician.
What should I do if I cannot turn off my main switch?
If the main switch is stiff, faulty or will not turn off, do not attempt to force it or work on the board. Call a registered electrician to inspect and replace the switch. A main switch that cannot be operated reliably is a safety concern.
Sources & further reading
- IET — BS 7671:2018+A2:2022 (18th Edition Wiring Regulations)
- Electrical Safety First — replacing a consumer unit best practice guide
- NICEIC — consumer unit and distribution board guidance
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.