Under what circumstances are the different methods used?

Butt fusion
Butt fusion can be used for jointing pipes and fittings of 90 mm OD (Outside Diameter) and larger. Some technical specifications allow the use of butt fusion down to 63 mm OD in carefully controlled conditions. Butt fusion jointing is equally suited to both  coiled and straight pipe lengths. The pipe clamps that form part of the welding machine are normally sufficient to deal with any ovality that may have occurred during storage or transportation. Only pipes and fittings of the same nominal diameter and end wall thickness may be butt fused together. Butt fusion can be used in new installations using traditional open cut trenching techniques. Additionally the low profile of the butt joint and its ability for the external bead to be removed, if required, make it ideal for use in trenchless technology installations such as  sliplining, pipe bursting, directional drilling, etc. Butt fusion can also be used to fabricate a range of fittings such as bends, tees and reducers. Such fabrication should be undertaken in a carefully controlled factory or workshop environment, using the correct equipment such as a fabrication table, and not on the construction site.

Electrofusion
Electrofusion is most commonly used for jointing pipes of up to 250mm OD, but there is no technical upper limit. It is nevertheless most commonly used for smaller diameter pipes because the cost of fittings increases with diameter. Electrofusion is equally suited to both coiled and straight pipe lengths and can be used to joint pipes of different nominal diameters and SDR's using suitable fittings. It should be emphasized however that pipe ends which are to be inserted in to the electrofusion socket must be circular in shape, within the tolerances given by the electrofusion fitting manufacturer. If the pipes ends have a high degree of ovality and are outside the allowed tolerances they must be rerounded using rerounding clamps and similar equipment. If the annulus between the outside of the pipe and inside face of the electrofusion fitting is not regular and out of tolerance this can lead to a bad joint which will have to be cut out and replaced. Electrofusion is most commonly used in conventional open cut trenching installation and in repair works. It is also used for adding new services to existing PE mains by means of saddle fittings or tees. It is less well suited to trenchless installation or rehabilitation methods because the fittings protrude significantly above the pipe outer surface. Electrofusion fittings are available in a wide range of forms; straight couplings, bends, tees, stop ends, reducers, saddle fittings, repair patches or repair fittings.

Mechanical
Mechanical assembly is used for jointing where neither fusion method is suitable. Most commonly this is when PE is being connected to other pipe materials or when PE pipe is being connected to valves or fittings made from other materials. Mechanical fittings are commonly used in repair situations, when fusion jointing is not possible due to wet and dirty conditions in the bottom of an excavation. They are also commonly used in remote areas where perhaps no power supply is available and/or operators with the required training and knowledge cannot be found. There are often restrictions on the use of mechanical fittings in gas networks, where fusion jointing is preferred in order to minimise the risk of leakage, but they are commonly used in water networks of all types.