Mention putting an electric tramway in the street and before very long the Statutory Undertakers will complain that stray currents will reduce their assets to rust in short order. Unfortunately, the Transport and Works Acts in force in the UK mean that the objections raised by the Undertakers are often given undue consideration. The Undertakers have exploited this situation to extract onerous – and expensive – mitigation measures from the tramway promoter.
The reason for this apprehension comes from the working of Ohm’s Law. All electrical conductors possess some degree of resistance to the flow of electric current. Voltage, or the electrical potential difference between any two points on the conductor, is necessary to get current to flow and is proportionate to the magnitude of the current flow. An electrical conductor is analogous to a water pipe – the greater the pressure differential between the two ends of the pipe, the faster the water will flow. Similarly, the greater the bore of the pipe, the lower the resistance to the water flow, so either more will flow for the same pressure difference, or the same amount will flow for a lower effort.
In scientific terms, this is defined as V = IR, where V is the potential difference measured across the conductor in units of volts, I is the current through the conductor in units of amperes, and R is the resistance of the conductor in units of ohms. That is, the current flow is equal to the voltage (pressure) divided by the resistance to flow.
Examining the issue
On any electric railway, current will flow outward to the train via the contact wire or the conductor rail. In doing so, some of the voltage (pressure) is lost in overcoming the resistance of the conductor: resistive voltage drop. The result is that the voltage at the train is lower than that at the substation. The current then has to complete the electrical circuit by flowing back to its source, experiencing yet more resistive voltage drop.
Electricity, like water, will always take the route of least resistance. On the way to the train it has no choice, as the live conductor is insulated, but on the return trip the conductor is not necessarily fully insulated. The track is quite poorly insulated from the ground; on street tramways, it may even be partly embedded in it. Although the earth is not an efficient conductor it is a three-dimensional body which means that there are an infinite number of potential current paths. As a result it forms a relatively poor insulator and although most of the traction current will return via the running rails, some will leak back to the substation through the earth. This is termed stray current.
The problem with stray current is that it, too, likes to find the easiest way back home and the ground in urban areas is littered with metal pipes and metal-sheathed cables belonging to the various Statutory Undertakers. So if a utility pipe presents itself as a lower resistance route the stray current will follow it. The problem arises where the current decides to leave the metallic equipment. With the DC electrifications used for tramways, the current flow is always in the same direction, as a result of which the metal undergoes progressive electrolytic corrosion and pipes will eventually leak at the exit point.
Managing stray current
Stray current is an issue that can be managed through intelligent design. When designing a tramway it seems simplest to connect the negative terminal of the supply to Earth, i.e. to a copper rod buried in the ground at the substation. From a stray current point of view though this is not a good thing, as it provides an easy return path for stray current that has travelled through Undertakers’ equipment. Leaving the running rails unearthed has the benefit of reducing stray current, as now the current has to leak out of the rails at the far end of the circuit and leak back in again nearer the substation to reach the negative terminal of the supply.
If the rails are imperfectly insulated from the ground, they will ‘float’ electrically, so that the average potential difference is zero. In other words, the voltage gradient from the train to the substation will start off positive relative to Earth, fall through a zero point nominally midway between the train and the substation, and end up negative at the substation. The effect is that the rail/earth potential is effectively halved, which will in turn halve the current.
Clearly, it is beneficial to keep the rail/earth voltage as low as reasonably practicable, given that the current is proportional to the voltage. One way to do this is to ensure that the resistance of the return circuit, i.e. the running rails, is low by maximising the cross-sectional area of the rail and ensuring that all joints are either welded or provided with suitably sized bonding cables. Resistance can be further reduced by cross-bonding between the rails and between tracks at regular intervals, connecting all the rails in parallel. Another useful measure is to avoid too great a distance between the substations, since the linear resistance of the track is proportional to the distance the current has to travel.
There is an interesting argument to be had in determining the design of the system as to whether to go for the railway-type approach with each electrical section of contact wire fed at each end from two substations; or to feed each length from a single point at the middle. The latter is practised by some European tramways: it ensures that the length of the return path is never more than half the interval between substations, but has the disadvantages that should the substation fail, a complete section becomes de-energised, necessitating end-feeding connections to adjoining sections to be built into the system to cope with failures.
It should also be appreciated that differences in the local high voltage supply can result in the share of load taken by the two substations in a double-end arrangement to be significantly unbalanced.
Another element of managing the effects of stray current is to maintain a non-conducting barrier between the rails and the earth. Whereas the first-generation tramway engineers could do nothing except try to keep the return circuit voltage drop within bounds, ‘wonder’ materials in the shape of pourable synthetic polymers had been invented by the time the Manchester Metrolink system was designed and were seized upon as a means of insulating the rails from Earth.
These materials promised excellent insulating properties and earth leakage resistances in the order of 1-200 ohm-km per kilometre of track were promised. In practice, these levels were never achieved.
Embedding the rails in polymer can pose significant problems for maintenance, as it effectively prevents access to the rails when something does go wrong. Even track renewal becomes more difficult. A widely-used approach in the rest of Europe is to employ a rubber pad under the foot of the rail, supplemented by rubber blocks tucked into the web of the rail on both sides; the rail itself is held down by more conventional means, such as baseplates cast in situ or embedded concrete sleepers. These both reduce the amount of rail in contact with the surrounding street material and provide a flexible vertical joint between the rail and the street.
How much is this an issue?
It is debatable to what extent stray current damage has actually been an issue in the UK. The author’s experience on the Croydon system was that, on the whole, the fears of the Undertakers were unfounded. The one place where a problem arose was where a lead-covered telephone cable in a buried iron duct route cut across a corner of the tramway on an off-street.
Tramlink convened a Stray Current Working Party (SCWP) throughout the construction and early operating phases. Once the system was up and running very few significant incidents were brought before the SCWP, and the Statutory Undertakers’ interest rapidly waned.
The lessons to be drawn from experience can be summarised as:
- Simple options often give acceptable results
- Anxiety is often over-played.
James Snowdon I.Eng., FIET, FIMechE was Chief Engineer at Tramtrack Croydon Ltd, from 1997 to 2008. Thanks also to Scott McIntosh.