2000

AS$8 million machine spurious start-up caused by transients

We were close to finishing the construction of an eight-million-dollar mining machine in a cavern in Australia. The operators of the mine had a central control room from which they wished to be able to exert manual control over any machine in the mine, even though the machines were automatic or had local control. Accordingly, the mine operators ran their own cables from their control room and connected them to spare inputs and outputs on the PLC for each new machine, also making the necessary software modifications themselves.

Suddenly, while we were standing by the machine, it started up of its own accord. Luckily, no-one was working on it at the time, although they could have been, but it was still a very serious issue as the machine had not yet been filled with lubricant and could easily have been wrecked. It turned out that no special precautions had been taken with the cables from the control room to the PLC, or with the software changes, and a transient interference with the new cables had caused our machine to start up unexpectedly.

1998

Chlorine gas release caused by mains transients

The HSE recently prosecuted the supplier of an item of equipment which 1ed to a release of chlorine in a semiconductor plant. The equipment was not sufficiently immune to mains transients (and proven to be so by the HSE's own labs). They were prosecuted under section 6 of the Health and Safety at Work Act because the supplier, though aware of the problem, did not inform the users of the equipment.

The case referred to was heard at Swindon Magistrates Court on November 25th 1998. The defendant entered a plea of guilty to a charge brought under S6 (1) (d) of the Health & Safety at work etc. Act, 1974. The magistrates imposed a fine of £5,000 and made an order for the defendants to contribute £7,000 towards HSE's costs of £9,482. The case concerned a microprocessor based valve control panel used to control the flows of chlorine and nitrogen in a semiconductor plant. There had been a release of chlorine resulting from all of the valves in the control cabinet being set to an open position.

Investigation by the HSE found that the unit was susceptible to conducted transients on the mains supply. There were no precautions against electrical interference in the power supply and the microprocessor watchdog was not effective in ensuring a safe state following detection of a fault.

1998

CE marked 8kA DC motor drive causes severe interference with monitors

A very powerful (8,000 Amps) DC drive was recently purchased and installed in an industrial plant. It was contractually agreed that it would meet and be declared compliant to the EMC Directive. A control room was also required (like most modern control rooms it was full of PCs and CRT-based VDUs) and the drive manufacturer said that it could be installed near their drive cabinets. When the drive was operated the images on the VDUs were squashed into 50% or less of the screen width. It was possible to tell the direction and loading of the drive directly from the movement of the VDU images, which of course were completely unreadable. The magnetic fields caused by the drive were of the order of 235mT, and most CRT-based VDUs show image movement at greater than 1mT (1mT is approximately equal to 0.8 Amp/metre and to 10 milligauss).

The drive manufacturer claimed that his drive did meet the EMC Directive despite the fact that it caused interference with the control room VDUs. What they in fact meant was that it met the industrial generic standards, which do not include any limits for low frequency magnetic field emissions. They forgot that their EMC Declaration of Conformity binds them to not causing interference of any kind, and that compliance with a harmonised standard only gives a presumption of conformity.

The situation has been remedied by the use of LCD screens, which have only recently been available with a specification suitable for the SCADA system that was used. "Dog kennel" magnetic shields and active field cancellation devices were also investigated. The delay in the use of the control room was several months, and this had an impact on productivity far beyond the cost of the remedial measures.

1996

10MW power converters interfere with wired telephones over 12 miles away

To cope with increased North Sea oil production, two new pumping stations with 6MW adjustable speed induction motor drives were built and installed in Scotland, one in Netherly and one in Balbeggie. Soon after commissioning the local power utility and the telephone company received a flood of complaints. Geographically the complaints came from concentrated pockets spread over an area up to 12.5 miles away from the 33kV overhead supply lines feeding the drives. A payphone over 4 miles away from the power line was noisy enough to be almost unusable, whereas just across the street a householder's telephone was relatively unaffected. Other symptoms included loss of synchronisation on TV sets (rolling pictures) and ringing on the supply to fluorescent lighting circuits.

Although the drives had been designed to, and met the supply industry's G5/3 harmonic limits, the problems turned out to be with higher order harmonics than it covered, up to the 100th in fact (i.e. 5kHz). The problem became a public relations nightmare for all involved, and culminated in questions being raised at Government level. Remedial EMC work was urgently required and was in fact accomplished, although under extreme difficulties because the cost of any downtime of the oil pumping stations was so high.