By connecting and sealing the saddle to the outside of the pipe first, and only cutting through the wall afterwards, from inside the saddle.

The sequence is: prepare the main, fix the saddle to it - fused or clamped - and let it develop full strength. Connect the branch pipework to the saddle outlet and prove it. Then, with everything downstream sealed, drive the saddle's integral cutter through the pipe wall. Water or gas enters the branch, but nothing escapes, because the saddle and the branch are already closed to atmosphere.

The cutter is then withdrawn and usually retains the coupon it removed, so no piece of pipe wall is left loose in the main. In many designs the retracted cutter also acts as a valve, allowing the branch to be shut off again.

This is why the cutter is built into the fitting rather than being a separate tool - the fitting is the pressure boundary during the cut.

The same scraping discipline as any electrofusion joint, applied to a more awkward surface.

The area of the main under the saddle's fusion footprint must be scraped to remove the oxidised surface layer completely, over the full area the coil covers plus a margin. Because the main is usually already in the ground, that means scraping the crown of a fixed pipe in a trench, often with limited access - which is precisely why it gets done badly.

After scraping the surface must be clean, dry and untouched. Mud, water and hand contact all compromise the fusion.

The main must also be adequately supported and the saddle clamped firmly to it, in full contact, for the whole fusion and cooling period. A saddle that lifts even slightly from the pipe as the polymer melts leaves a void. Under-clamping is the second most common cause of saddle failure after inadequate scraping.

A branch substantially smaller than the main, with the permitted combinations published by the manufacturer.

The limit comes from the hole. Cutting through the pipe wall removes material from a pressure-carrying member, and the larger the hole relative to the main, the more the main's own capacity is affected. The saddle must also seal over a footprint that fits the pipe's curvature, and its cutter is sized accordingly.

Service connections - the usual application - are typically a small fraction of the main diameter, so this is rarely a constraint in practice.

Where a larger branch is required, the options are a full-bore tee fused into the main, which means taking the main out of service, or a purpose-designed large-branch under-pressure tee assembly, which is specialist equipment. Check the manufacturer's main-to-branch table before assuming a saddle will do the job.

Yes, and gas is one of their largest applications - but gas fittings, procedures and operator competence requirements are separate from water.

The principle is identical, and under-pressure tapping is standard practice for connecting gas services. But gas work is governed by its own regulatory regime, the fittings must be approved for gas, and the operators must hold the relevant competence for live gas operations. Purging and commissioning procedures apply that have no water equivalent.

Water fittings must never be used on gas, and the reverse should not be assumed either.

The pipe colour conventions exist partly to prevent this confusion, but colour varies between markets, so the printed pipe marking is the reliable identification. Confirm what the main actually is before working on it, and work to the gas regime where it applies. This is one of the few places in the range where getting it wrong is immediately dangerous.

How it attaches, and therefore where each is used.

An electrofusion saddle has a heating coil moulded into its contact face and is welded to the main, becoming part of the pipe. The joint is as permanent and as strong as any fusion joint, with nothing to loosen, and it is the standard for new PE service connections.

A mechanical or bolted saddle clamps around the pipe with a strap or a second half-shell and seals with a gasket against the pipe outside. It needs no power, no fusion equipment and no scraping, and it can be fitted to materials that cannot be fused - cast iron, ductile iron, PVC, asbestos cement - which makes it the practical choice on mixed and older networks.

Mechanical saddles are also used for repairs and where a connection may need to be removed. Their limitation is a gasket seal that depends on the pipe's outside surface being sound, and a bolted assembly that can relax over time.