permanent magnet so that no warning is received on the train. The suppressor magnet is fail-safe since loss of power will cause it to act like an ordinary permanent magnet. A cheaper alternative is the installation of a lineside sign that notifies the driver to cancel and ignore the warning. This sign is a blue square board with a white St Andrew's cross on it (or a yellow board with a black cross, if provided in conjunction with a temporary speed restriction). With mechanical signalling, the AWS system was installed only at distant signals but, with multi-aspect signalling, it is fitted at all main line signals. All signal aspects, except green, cause the horn to sound and the indicator disc to change to yellow on black. AWS equipment without electromagnets is fitted at locations where a caution signal is invariably required or where a temporary caution is needed (for example, a temporary speed restriction). This is a secondary advantage of the system because temporary AWS equipment need only contain a permanent magnet. No electrical connection or supply is needed. In this case, the caution indication in the cab will persist until the next green signal is encountered. To verify that the on-train equipment is functioning correctly motive power depot exit lines are fitted with a 'Shed Test Inductor' that produces a caution indication for vehicles entering service. Due to the low speed used on such lines the size of the track equipment is reduced from that found on the operational network. 'Standard Strength' magnets are used everywhere except in DC third rail electrification areas and are painted yellow. The minimum field strength to operate the on-train equipment is 2 milliteslas (measured above the track equipment casing). Typical track equipment produces a field of 5 mT (measured under the same conditions). Shed Test Inductors typically produce a field of 2.5 mT (measured under the same conditions). Where DC third rail electrification is installed 'Extra Strength' magnets are fitted and are painted green. This is because the current in the third rail produces a magnetic field of its own which would swamp the 'Standard Strength' magnets. Expansion of AWS application From 1971, an AWS permanent magnet was fitted ahead of the warning indicator where the permissible speed of a line dropped by more than a third. This was a recommendation of the inquiry into the derailment at Morpeth on 7 May 1969.