Suppression of the surges

Lightning may strike only once, but overvoltage transients from other sources occur frequently and can be equally damaging. This is why there is a strong case for the inclusion of relevant protective devices in electrical installations, especially those supplying power to electronic equipment and computers. And nowadays there are few industrial or commercial installations where this is an exception. Even if ups equipment is installed it is likely to be necessary to provide transient overvoltage suppressors.

For electrical contractors the installation of transient voltage surge suppressors (tvss), or surge protective devices (spds) as they are called by some manufacturers, provides new business opportunities. These opportunities have been boosted by the recent launch in the UK of a range of protective devices from the USA and another from an Austrian manufacturer. More about these later; first something about transient voltages.

Transient voltage surges

A transient voltage surge is a large increase in voltage lasting for no more than a few microseconds. The increase in voltage will vary from just a few volts to thousands. More than 80% of these transient voltage surges originate from inside the buildings themselves; only 20% are caused by external sources. Transient overvoltages are caused by electronic power devices such as switch mode power supplies in printers and other office machines, electronic control gear in fluorescent lighting, variable speed drives for lift motors and hvac plant, and welding plant. Equipment that can be affected by overvoltage transients can also be their cause.

Robotic failure

Studies undertaken in the USA have revealed that in a large industrial complex as many as 432 000 transient overvoltages can occur in an hour. Even in the relevant calm of commercial premises there might be between 9000 and 60 000 transient overvoltages in an hour. Most of these overvoltages will be slight and not immediately damaging, but they can have an accumulative eroding effect which can eventually lead to equipment failure.

A Ford assembly plant was placed in jeopardy through the constant failure of robotic equipment, due to the effects of transient overvoltages. The pcbs in the robots were being damaged, in some cases at the rate of four to eight per day.

The situation was remedied by a plant-wide surge suppression programme, including the fitting of a tvss to each robot. Payback was achieved in 38 days. In another plant, the switching off of a twin tube fluorescent fitting caused damaging transients of over 1200 V. These were eliminated through the installation of suppression devices. Although lightning is a lesser cause of transient overvoltages than other sources, protection is advisable.

To be effective a surge suppression installation provides four levels of protection – at the main switchboard, at the subdistribution boards, the final distribution boards and at the point of use by the equipment.

The different levels are necessary, as at the first three levels there are likely to be residual overvoltages which could be damaging to equipment at lower levels. TVSS units are categorised according to the location they are designed for. Confusingly, those made in Europe are categorised in the reverse order to those made in the USA.

MEM is now offering a range of tvss equipment made by American manufacturer Innovative Technology. Off-the-shelf equipment in the MEM Protector range comprises units for Category C locations (the main switchboard), Category B locations (subdistribution boards) and Category A locations (final distribution boards). There are also units for dedicated protection of critical equipment. The categorised locations are those defined in American standard, ANSI/ IEE C6.41.

By contrast, the devices now being offered by F&G Switchgear are categorised in line with the VDE standard, in which Class B relates to main switchboard protection, Class C distribution boards and Class D individual items of equipment. The standard also provides for a Class A location, which is for power line protection. There is at present no British Standard specifically covering transient voltage surge suppressors as such.