Life is Full of Ups & Downs - Part 12

ON REPAIRS - GRACE Building (Continued)

The Big Bang!

With a manually-operated lift, the makeup of the control-board was pretty simple

Apart from a couple of smaller contactors or relays for the door-lock & safety circuitry, there were two large, 480V DC contactors, one for Up and one for Down and which were operated by a solenoid.

Very simply, when the driver operated his car switch then the core of the solenoid was energised and the electromagnetism thus generated pulled the moving part of the contactor inwards and large copper contacts met up with large carbon contacts, the lift motor was energised and the lift moved in the appropriate direction.

As well as the essential Up and Down contactors, safety-circuit relays and fuses the board may also have featured a stepping-solenoid contactor set, which allowed the lift to accelerate smoothly.

A typical 480V DC lift controller of the era

The slate on which all this large, heavy and banging-in-and-out stuff was mounted was around an inch thick and part of the maintenance task was to wipe down any carbon-dust, not just because of cleanliness but also because a buildup of dust could lead to electrical "tracking" between components.

One afternoon, Bobby Coates, whose maintenance depot was Grace Building, was standing in front of one of the control-boards with another mechanic (I don't recall who it was) and they were looking at a crack which was just noticable on one side of the board, starting at one of the direction contacts.

Bobby was probing the crack with a piece of stiff plastic or something similar....whatever it was it was most certainly not electrically conductive....as he and the other bod investigated to see how far it went and to where.

Suddenly there was an almighty BANG! accompanied by a bright green flash which lit up the motor-room. I was on the other side of the room and even from that distance my ears were ringing!

Bobby and whoever-it-was staggered back...fortunately, because it was away from the board and into a recess where a small table and a couple of seats were situated.

They were blinded for a minute or so, also deafened and with carbon dust on their faces and were extremely lucky not to have been burnt.

Naturally the horrible (I hated those things with a passion!) open-air circuit breaker (located on the wall next to the motor-room access steps) for that lift had blown....its BANG! and green arc also adding to the noise and flash produced by the controller.

When Bobby and the other bloke had recovered and the lift was isolated and removed from service it was discovered that the crack, fine though it was, ran from one of the fixed contacts on the Up contactor across to a fixed contact on the adjacent Down contactor and it contained carbon dust...an excellent conductor of electricity.

When the lift driver operated the car-switch 480 volts of direct current travelled from Up to Down, causing a very spectacular short-circuit.

If memory serves, that lift was out of action until the crack in the slate was enlarged, thoroughly cleaned out and filled with epoxy as a temporary repair. I think that at a later date a new, composite (not slate) board was installed.