IN an apartment block 39 storeys high, the lifts are an essential part of the building. Even if one is temporarily out of action, the passenger flow can get clogged up. The Housing Department has the headache of overseeing about 5,100 lifts, as well as 150 escalators in its commercial centres. On average, there are 1,900 breakdowns a month, which means each lift goes nearly three months without a breakdown, a better rate than the private sector can lay claim to according to Vincent Tong, the Housing Department's chief building services engineer. In fact, the breakdown rate could be considerably better. ''Around 45 per cent of failures are due to vandalism or misuse,'' he said. ''There were only three incidents in 1992/93 which involved major equipment failure or personal accident.'' As lift companies install and maintain their own equipment, the Housing Department has introduced strict criteria which each contractor has to meet before he is allowed to tender for work. A low lift breakdown rate is one requirement, but the length of time for which it is out of action is equally important. Contractors are expected to be able to rescue anyone trapped in a lift within 30 minutes. This will undoubtedly seem a long time to the victim but, as Mr Tong pointed out, contractors cannot be expected to have a depot on every estate, and Hong Kong's traffic situation makes it difficult to guarantee a faster turnout. A third factor to be considered when assessing the contractor is the quality of service. A Housing Department inspector routinely checks both the weekly service work of the contractor, and the six-monthly major service. As a result of this tight control over standards, 10 lift companies have been listed as approved contractors, and are asked to tender for public housing work. The Housing Department is also undertaking a modernisation programme. Mr Tong concedes that in the older estates the lift service is inadequate in terms of capacity and speed. Except for those in buildings scheduled for redevelopment, all lifts which have been in operation for more than 20 years are being replaced. More than 100 have been replaced so far, but Mr Tong has asked for an extra $30 million to speed up the process. There is also an ongoing programme to improve lifts by replacing motors, control equipment and doors, and upgrade safety arrangements. The latest Harmony blocks contain six lifts, each with a capacity of 15 people. Even with this number, the maximum waiting time may be as long as five minutes because of the height of the building and the number of occupants. But it is difficult to add extra lifts to old buildings. They either have to operate in a new external shaft or replace living areas.