Big Ben is famous for its reliability.
This is due to the skill of its designer, the lawyer and amateur horologist Edmund Beckett Denison, later Lord Grimthorpe.
Together with an enclosed, wind-proof box sunk beneath the clockroom, the Great Clock’s pendulum is well isolated from external factors like snow, ice and pigeons on the clock hands, and keeps remarkably accurate time.
Despite heavy bombing the clock ran accurately throughout the Blitz. It slowed down on New Year’s Eve 1962 due to heavy snow, causing it to chime in the new year 10 minutes late.
The clock had its first and only major breakdown in 1976. The chiming mechanism broke due to metal fatigue on 5 August 1976, and was reactivated again on 9 May 1977. During this time BBC Radio 4 had to make do with the pips.
It stopped on 30 April 1997, the day before the general election, and again three weeks later.
On Friday, 27 May 2005, the clock stopped ticking at 10.07 p.m., possibly due to hot weather (temperatures in London had reached an unseasonal 31.8 °C/90 °F). It resumed keeping time, but stalled again at 10.20 p.m. and remained still for about 90 minutes before starting up again.
On 29 October 2005, the mechanism was stopped for approximately 33 hours so that the clock and its chimes could be worked on. It was the lengthiest maintenance shutdown in 22 years.