Electrical faults can be divided into two categories, supply system faults, which affect all trains in the control area, and train faults, which affect only individual vehicles. This is distinct from faults arising from derailments where a combination of track and vehicle defects are the cause, which would not occur if only one or the other were present.

The fault diagnosis procedures described in this section can be carried out using only a test lamp. A 12V 21W car lamp taking about 1.75A will be suitable in most cases. Use two in series for 24V systems. If, when placed across the power supply output, the lamp load causes the power unit protection to operate, reduce the lamp wattage. For example, the test lamp described above would cause a small commercial unit rated at, say, 12V 1A output to trip. A lamp rated at 12W would be suitable for this size of unit (Figure 1).

Although not essential for tracing faults, panel mounted or portable meters will give more information about a fault than a test lamp as well as monitoring the system under normal operating conditions.Power supply systems includes the installation of meters and interpreting their readings.


Figure 1. Test equipment. The lamp on the left is a 21W car lamp with approximately 400mm of cable and test clips. The short lengths of rod held in the test clips temporarily convert them into probes when checking circuits connected by block connectors. The centre test lamp is a torch bulb in a batten holder used for a quick check on track power when first setting up a portable test track or layout. One terminal is connected to a strip of aluminium by a 100 ohm resistor to limit the current through the bulb, while the other has about 70mm of lead terminating in a simple probe. The strip is roughly hook shaped, which makes it easy to place it in contact with one running rail while the probe is touched to the other. A useful item is a pair of jumper leads fitted with clips to bridge a faulty joint until repairs can be carried out.

This indicates a locomotive or rolling stock fault. Refer to Train fault diagnosis below.

A short circuit will cause the power unit protection to trip.

• Insert the test lamp in to the circuit at the power unit or controller output and turn the controller full on (Figure 2). If the lamp flashes and the protection trips, the fault is in the power unit or controller. If the fault is elsewhere the lamp will light, its brilliance depending on the resistance of the short.
• Open the switch of the section on which the train is standing.
• If, when the switch is opened, the short disappears, first check that the train is not bridging sections of the opposite polarity then lift it off the track and reclose the switch.
• If the fault does not reappear the fault is in the train. Replace each vehicle one by one. If the short reappears, the fault is in the last vehicle to be replaced. Refer to Train fault diagnosis below.
• If, when the switch is opened, the short persists, the fault is not in the section on which the train was standing. Isolate the sections one by one until the faulty one is found. If the fault persists when all sections are isolated, the fault is in a part of the circuit between the power unit and the control panel which remains live.

Figure 2.

Insert the test lamp into the circuit at the power unit or controller output as described above and shown in Figure 2. Remove all loads and energise the system. If the test lamp glows proceed as for locating a full short circuit.

On large outdoor railways there may be some current leakage particularly when the track is wet. This is unavoidable and the power unit should be capable of supplying the system without unacceptable voltage drop. On some large systems leakage currents of up to 0.5A have been noted.

• If all trains slow down significantly as they travel away from a track supply connection with the controller on a constant setting, the fault is in the supply system.
• If a particular locomotive slows down as it proceeds away from a supply point it is drawing too large a current for the supply system. This could be due to a motor fault or because it is fitted with an old type motor having a high current consumption.
• If all trains do not respond to the controller as expected there could be a high resistance between the power unit and the track feeder connection. With series resistance control units a partial short can show the same symptoms. If this is suspected test as described for a partial short circuit above.
• If a locomotive performs poorly irrespective of its position on the track the locomotive is defective.

Using an Ohmmeter or a portable multimeter is the best method of tracing high resistance in the supply system but a test lamp can give some indication of its presence. The procedure is initially to remove all load from the system and connect the test lamp clips across the power unit output. Set the controller to the full speed setting and then move the test lamp clips progressively along the circuit. There will be a gradual reduction in brightness due to voltage drop in the wiring and rails but if it is excessive refer to the table below for possible causes. A sudden decrease in brightness will reveal a local fault that needs attention.

Train faults can be separated into short circuits caused by rolling stock defects and locomotive faults that are more complex. The latter can occur in the construction of the locomotive or in the motor itself.

The following actions assume that the motor internals are accessible for repair. Otherwise a replacement motor would be required.

When a layout has been built, tested and operated successfully for some time, a fault can initially cause some confusion. By systematically checking as described above, most of the causes can be located. However, occasionally two or more faults can occur at the same time and these can troublesome to separate and identify. The following case histories may assist in highlighting some of the difficulties encountered.

A 4-4-0 locomotive running at a moderate speed on an oval test track kept stopping at the same point. A slight push would set it off again and it would run freely until the same point was reached. The track was examined and a slight dip in one rail was noted. The locomotive pick-ups were of the rim-scraper variety operating on the driving wheels only. When examined, one was found to be bent and out of contact with the wheel. As the underframe was a rigid type, the single wheel still conducting on that side was lifted clear of the rail at the dip, breaking the electrical contact and stopping the locomotive.

A locomotive approaching a trailing turnout came to a halt. A short circuit was noted and the locating procedure described above was carried out. The short circuit no longer appeared so the locomotive was put back on the track and restarted. When it reached the same position, the section gap before the point crossing, the short reappeared. With the power on and the locomotive lifted off the track, a check using a test lamp showed that, although the turnout was correctly set, the crossing polarity was wrong. The crossing was fed by a tiebar operated micro-switch and an examination showed that the pad that pressed on the switch button had broken away and the switch was not being operated.

Two trains in adjacent sidings in a fiddle yard moved off together almost colliding at the fouling point when they were noticed and switched off. The turnouts had live crossings fed via the point blades and were used to isolate the sidings see Point wiring. The blade that should have made an electrical contact with the stock rail was prevented from doing this by a fine piece of grit. As a result both locomotives were connected in series. Figure 4 illustrates the problem.


Figure 4. Backstairs or ‘sneak’ circuit.

A six-coupled locomotive was being tested on a temporary piece of track which included a turnout. The turnout was a commercial product with an insulated crossing. As the locomotive passed over the turnout it stalled if driven slowly but was satisfactory at speed. Each time it stalled it was with the centre driver on the insulated crossing. A bench test showed that all drivers were picking up correctly and no fault was found. It was finally realised that the plastic crossing was a solid moulding and the flange way was quite shallow. The flanges of the drivers were riding on the moulding and, as the frame was a rigid type, slightly lifting that side of the locomotive. When the centre driver reached the crossing it lifted sufficiently to break the contact of the other two drivers and, at slow speed, the locomotive was brought to a halt.

Locomotives on a large garden railway performed poorly over a long stretch of the main line. Checking the voltage at the controller output and on the track with no locomotives present gave similar readings. When a locomotive was placed on the track, the track voltage fell to below half the controller output voltage. The cause was eventually traced to a dry joint in the feeder connection to the track.

This article was compiled for the Gauge O Guild Manual. It was adapted for the GOGWiki by Nick Baines.