Because polyethylene oxidises at its surface, and fusing to an oxidised layer produces a weak joint no matter how perfect the fusion cycle looks.

Exposed to air and sunlight, the outer skin of PE pipe degrades. That layer is only a fraction of a millimetre thick and is completely invisible, but it does not fuse properly to the melting polymer from the fitting. The joint can pass its fusion cycle, show correct indicators, hold a pressure test and fail months later.

Scraping must remove the full surface layer over the entire circumference and the whole insertion depth - not a light abrasion, and not just the areas that look dirty. A purpose-made rotary or peeler scraper is far more reliable than a hand blade, which tends to miss the underside.

After scraping, do not touch the surface. Handle the pipe elsewhere, wipe with an approved cleaner if required, let it dry fully, and assemble promptly. Re-scrape if the joint is not made straight away.

Wherever the pipes cannot be brought into a machine and aligned - which covers most repairs, tie-ins and confined-space work.

A butt fusion machine requires both pipe ends free to move, clamped and aligned, plus room to operate the carriage and withdraw the heater plate. In a narrow trench, against a wall, between two fixed connections, or when repairing a pipe already laid, none of that is available.

Electrofusion needs only enough clearance to slide the fitting over the pipe end, and the pipes can be effectively stationary.

It is also preferred on smaller diameters where the cost of a coupler is modest, for branch connections using electrofusion saddles, and where the traceability of the barcoded cycle record is required.

Butt fusion remains better for long open runs and large diameters, where the absence of any fitting cost and the clear bore are worth the machine time.

It carries the fusion parameters for that specific fitting, so the control unit sets the cycle rather than the operator.

Each fitting size and type requires a particular voltage, current and fusion time. Historically these were entered by hand, and entering the wrong figures produced under- or over-fused joints. The barcode removes that: the operator scans it with the control unit's wand and the unit configures itself.

Modern units also record what happened - the fitting details, the actual voltage and current delivered, the ambient temperature, the operator's identity and a timestamp - and store it for download. That log is the traceability record for the joint, and on utility work it is usually a contractual requirement.

The unit will also abort and record a fault if resistance is out of range, which catches a damaged coil or a poor terminal connection before the joint is made rather than after.

The pipe must not move, and the cooling time must be observed in full.

During fusion the polymer inside the fitting is molten. Any movement of the pipe - a nudge, a tug on the cable, ground settlement, someone leaning on it - disturbs the melt and can create a void or a cold spot. Alignment clamps should be used to hold both pipes and the fitting rigidly, and left in place.

After the cycle ends the joint is still molten inside and has no strength. The published cooling time must elapse before the clamps come off, and a longer period again before the joint is pressurised or the pipe is handled. Rushing this is a frequent cause of failure, particularly under programme pressure.

Most fittings have melt indicators that rise when the cycle has run, but these confirm only that fusion occurred - they say nothing about scraping, contamination or movement, and should not be read as a quality check.

They can, but conditions must be controlled and the parameters may change.

Water is the main enemy. The joint surfaces must be dry at fusion - water on the pipe or inside the fitting flashes to steam and creates voids. Trenches must be dewatered and the joint area kept dry with a tent or shelter in rain. A fitting that has been submerged should not be used.

Cold affects the cycle. Fusion parameters are specified against ambient temperature, and below a stated threshold either an adjusted cycle or pre-heating of the joint area is required. Cold also makes coiled pipe stiffer and more oval, so re-rounding clamps become more important.

Direct sunlight causes the opposite problem: one side of a pipe heated by the sun and the other in shade fuses unevenly, so shading the joint is worthwhile on hot days.

Follow the fitting manufacturer's environmental limits rather than working to a general rule.