What happens when the Alarm Chain (Emergency Chain) is pulled in a coach? (ACP, Alarm Chain Pulling)

July 17, 2019, 1:59 PM

The alarm chain in a passenger coach is designed to create a break in the continuity of the brake pipes (whether vacuum or air brakes), immediately resulting in a loss of brake pressure (or vacuum) and thereby cause the train brakes to be applied. With vacuum brakes, a clappet valve is provided that is released by the pulling of the alarm chain, and with air brakes, there is a similar passenger emergency valve that can vent the brake pipe to the atmosphere. See the page on IR brake systems.

At the locomotive, in addition to a warning lamp or buzzer being sounded, in most locos the master controller undergoes auto-regression, with the notches falling to zero rapidly as the locomotive’s motive power is switched off. The guard may also notice the loss of brake pressure (although he may not know it is due to the pulling of the alarm chain) and is expected to apply his brakes as well immediately. It is possible for a driver to override the alarm chain pull in a few circumstances, and this is in fact done in a few cases where it is known that miscreants resort to pulling the emergency chain solely to get the train to stop at a point convenient for themselves (but note that such an act by the driver or guard of deliberately ignoring an indication of alarm chain pulling is a serious offence).

In recent years, locos have been fitted with emergency flashers on the roof of the cab, and these flashers are also activated when the brake pipe pressure is lost for any reason other than the driver’s application of the brake valve (A9). This alerts drivers of oncoming trains of the possibility of a derailed or parted rake which may foul other tracks, since the brake pressure may have been lost for those reasons as well, and at the locomotive it is not possible to tell whether the loss of brake pressure is due to the pulling of the alarm chain.

ACP also causes a small lever to be released near the emergency brake valve (usually mounted near one end of the coach) which does not retract to its normal position even when the chain is released. This allows the driver or guard to find out in which coach the ACP actually occurred. When the coach is isolated, the lever needs to be manually reset. Until this is done, the lamp and buzzer in the locomotive cab are continuously activated. A circuit breaker controls the lamp and alarm bell in the locomotive cab; in cases where defective equipment causes the lamp and bell to go off, the driver can disable them by placing the MCB in the ‘off’ position; despite the obvious safety hazards, sometimes this is resorted to by drivers when driving trains through sections where spurious ACP incidents ae very common.

Source – IFRCA.org

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