Street life

Maintaining street lighting is an extremely important responsibility for local and district councils. To maintain a safe environment at night and to minimise disruption during rectification, the fast and accurate location of street lighting cable faults is essential.

Pre-testing

Cable faults in street lighting take two forms: conductor faults and insulation faults. Before commencing fault location on a street lighting network, all fuses and neutral links should be removed so that the cable is isolated.

Removal of the neutral links is essential, because if the neutral is not damaged it can be used as the sound return conductor for insulation fault location. Also, if fuses are left in, any tests carried out will give misleading results.

Conventionally the first step in fault location is to conduct tests to check insulation and conductor continuity. While it is easy to test insulation between the phases and neutral from the feeder pillar, checking continuity from the feeder pillar to each load on a "tee" network can be time consuming, particularly if there is no record of which phase feeds each load. On a "looped" system checking continuity is obviously simpler and the results of these tests indicate what type of cable fault locator should be used.

An open circuit is often indicated by the fact that some lamps are not illuminated but no fuse has blown. In this case, an instrument such as the Lexxi T810 can be used to look for the fault, either from the feeder pillar, or from the lamps on either side of the open circuit – the last lit lamp and the first unlit lamp on the faulty phase.

It is essential to remember that fault diagnosis will give the most important clue to deciding which instrument to apply, and testing for continuity is best done with a simple multi-meter and not an insulation tester. Time domain reflectors will find low resistance shunt faults up to about 300 Ω and high resistance series faults over about 300 Ω. Beyond these limits an instrument such as Bicotest's T272 fault indicator should be applied.

Cable fault location

If one or more phases are down but the neutral is healthy a conventional Murray loop test (see figure 1) can be conducted using a fault locator such as Bicotest's T272.

The test can be made from the feeder pillar with the faulty phase and neutral connected at the furthest lamp on that phase. When the fault locator is "balanced", the distance to the fault will be indicated as a percentage of the loop length.

When carrying out a bridge test, a fault anywhere along the tee will appear to be at the tee joint. And so, if a bridge indicates that a fault is close to a major tee, the bridge test should be repeated with a loop at the end of that tee to prove whether the fault is at the tee joint or along the tee.

This procedure can be carried out on other faulty phases to ascertain whether there is more than one fault.

If one phase and neutral are down with two phases healthy, the Hilborn loop method (see figure 2) can be used. This utilises the two healthy phases. When the bridge is balanced, the reading will give the fault position as a percentage of the cable length.

When all phases and the neutral are down the Hilborn loop is utilised again, but since there are no sound conductors, a special "overland" lead is used.

This is a twin lead which is used to span the gap between two lamps on the same phase. The conductor cross-section should be about 1mm². The more lamps that can be spanned at one go, the easier the fault location will be, but practical limitations with respect to handling the lead means that 200-300 m is probably the maximum length.

Fault location on a looped network

A looped network is one where a single phase cable is used. The cable runs from the feeder pillar into the first lamp and then from the first lamp to second lamp and so on. The cable loops in and out of each lamp, rather than having a tee joint from a main cable to each lamp, often there are subsidiary loops to illuminated bollards and signs.

An open circuit could be located from the feeder pillar or from the lamps on either side. If the neutral is healthy, an insulation fault location could be carried out using a Murray loop on the complete length of the cable.

If both phase and neutral are faulty, then it is best to sectionalise the cable to find which length is faulty. This is done by breaking the connection at halfway and checking the insulation at both lengths of cable, then repeating this until the fault is known to lie between two lamps. A Hilborn loop test could then be carried out on this short length of cable.

Many looped systems use a split core concentric cable, where the neutral consists of a number of individually insulated conductors. Often by separating the neutral wires at both ends of the faulty section, a fault can be found without the need for an overland lead. Sometimes a check on each individual neutral to earth will indicate an obvious difference.

If this is not successful then a check on the insulation resistance between the individual neutrals and earth will often show that one or two have a much higher resistance than the rest. This will allow a Murray loop location to be carried out on one of the faulty neutral wires, or, if two neutrals have a high insulation resistance, a Hilborn loop location can be carried out on the line conductor.