Common causes of lift electrical failure

Jon Lawson

Gary Morgan recalls instances of electrical interference in lift systems that are indicative of problems that persist to this day

A passenger lift is essentially a small, metal box with limited airflow, suspended many metres in the air by nothing more than a hoist.

When you think of it like that, it is hardly surprising that so many of us harbour phobias of elevators. One of the most pressing threats is from electrical faults.

In my experience supplying elevator control systems to businesses worldwide, two of the worst instances of electrical faults were a Leeds, UK, shopping centre and a big manufacturing company in Germany.

These two incidents highlight the typical electromagnetic compatibility (EMC) issues that electrical contractors must tackle on elevator-related projects.

Retail environments

The shopping centre was a four-car variable frequency (VF) motor-drive group of elevators that had been working fine for three years, before then blowing a £3,000 central traffic dispatching computer.

After spending three days on site, I found 180 earth faults — which was a shock as this installation had been checked for earth loop impedance by a reputable engineering company and passed with flying colours.

After another £3,000 worth of kit failed, I went back and, to my horror, found the five inch mains riser was terminated to a brand new distribution panel being installed while I was on site. The riser went into a gland plate that sat on a cork gasket, nylon insulation washers and powder coated metalwork.

There was no earth conductor at all.

There are two problems with this. The first is that if we suffered a secondary fault, such as a door lock short to earth, then the elevator could run with the doors open. Secondly, the DC bus rises to 600V on each VF drive and could have proved fatal to the users pushing buttons outside the lift.

Industrial environments

The second interesting site was one for a big manufacturing company in Germany.

A factory the size of a car plant with automated trains and conveyors would dump it's PLC software on a weekly basis. The company spent a fortune sending engineers out to re-program these huge machines for six months

The problem was that the main control system was fed from a supply on the other end of the factory that already had an earth conductor and they didn't want the expense of installing a second one.

Unfortunately, this earth conductor turned out to be the roof lightning conductor so the only option was for the German electrical contractors to dig floors up.

Since these incidents, I've been doing EMC training for customers. All these training days bring out the same stories.

For example, a tachometer fault will be the result of poor bonding of the trunking runs and pigtails on the screen for the tachometer. From my experience, the two biggest problems are pigtails on the hoist motor terminals combined with poor bonding of the trunking runs.

Pigtailing cable screens has been deplored by EMC experts and IEC 61000-5-2 for many years, but nevertheless electrical contractors still do it.

How long will it take to retrain them all, so that fixed installations in Europe stand any chance of complying with 2004/108/EC regulations? Of course, this is not the only issue!

While mechanical failure of elevators is the key concern for consumers, contractors must keep in mind the importance of effective EMC installation and the problems associated with bad practice such as pigtailing cables.

To ensure safety in both consumer and industrial environments, electrical contractors need to understand and abide by EMC guidelines.

For advice on EMC best practice, as well as access to a number of educational resources, visit the EMC Standards.

Gary Morgan is with Liftstore Ltd.