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Electrical safety assured.

Safety and security are two high priorities for installers and users of electrical switchgear. To prevent personal injury or property damage as a result of issues such as fault currents (short circuits), a number of industry standards and regulations have to be followed.

These also cover the potential equalisation between all the metallic parts of an enclosure.

The TS 8 baying system from Rittal is an example of how safety can be built-in through automatic potential equalisation without necessarily having to separately earth each individual panel.

The earthing of metallic parts on electrical systems for safety reasons is prescribed virtually everywhere. This applies to all electrical equipment and units--from simple lamps to low voltage distribution systems.

The reason is quite clear: if a problem occurs (for example, an enclosure becomes live) it has the potential to cause serious harm. In low voltage switchgear, all the metal frame and enclosure parts endangered by stray voltage have to be earthed.

Attaching the earthing straps

To this end, earthing straps of copper wire are usually provided. These are flexible and therefore can safely contact moving parts (such as doors) to the enclosure. In enclosures, earthing straps typically also have to be connected to the frames, the side panels, the enclosure roof and any other panels.

Once all the earthing straps have been correctly fitted, there is an equalisation of potential, and all the enclosure components can be earthed via the protective conductor of the voltage supply.

The earthing straps have to be attached to enclosures by hand during their assembly, creating the potential for human error. Should a strap be forgotten, the finished switchgear will still be able to function despite the danger it might pose to people in the event of a fault.

Potential equalisation even without an earthing strip

To help limit this risk, Rittal has developed an effective solution with the TS 8 bayed enclosure system.

During the assembly phase, the side panels, enclosure roof, rear panel and gland plates are automatically connected to the frame conductively. This means potential equalisation can be achieved without necessarily having to connect these parts with earthing straps (although the final decision as to whether to leave earthing strips out completely is up to the user).

The solution is implemented using special claws or washers, which press through the electrically non-conductive surface coating of the panels during assem-bly, to achieve a reliable contact. It means that all the flat panels are electrically connected to the frame, so that--in many applications earthing straps are no longer needed. Instead, the earthing strap only needs to be attached to the enclosure door.

The contact resistance between the panels and the enclosure frame is less than 0.1 Q, the value demanded by the DIN EN 62208 empty enclosures standard.

Simplified installation and greater safety

This automatic potential equalisation reduces the amount of material needed and enables easier assembly, simply because the time-consuming attachment of up to seven earthing straps per enclosure is no longer necessary. It is also easier to disassemble and re-assemble individual panels because the earthing straps do not have to be removed and re-attached each time.

Users must consider, during a risk assessment, whether the design measure of automatic potential compensation is sufficient or whether separate earthing straps are needed to meet the technical safety requirements.



The answer to this month's trouble shooter is provided by MISTRAS Group Ltd.

Q How can I assess and monitor the condition of bearings that rotate at speeds of between 0.5 and 60 RPM?

A At lower bearing speeds (i.e. between 0.5 and 60 RPM) a number of proven inspection methodologies and advanced diagnostics are available to rapidly assess the condition of rotating or reciprocating bearings during normal operation. Acoustic emissions technology, is particularly effective on high value capital plants and machinery, where the cost of any lost production or machine downtime due to bearing failures is high.

These techniques have been successfully used in thousands of field tests on a variety of machines, plants and structures, including pressure vessels, power transformers, bridges, process storage tanks, pipelines, valves, nuclear lift rigs, railroad tank cars, cranes, Basic Oxygen Steelmaking (BOS) vessels and compressed gas cylinders.

A variety of bearing defects can be monitored, including corrosion of tracks or balls, insufficient lubricant, cracking or plastic deformation of bearing material, crushing and fracture of debris in the bearing. These techniques are even effective at assessing the condition of bearings that rotate at less than one revolution per minute.

Low-speed rotating or reciprocating bearing signatures can be produced after a few dozen assessment cycles. A more detailed analysis would involve the identification and elimination of extraneous noise, grading the severity of the bearing signature, and identifying any features that may indicate the root cause of the problem.

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Title Annotation:Maintenance Matters: Problem Solver
Publication:Plant & Works Engineering
Date:Apr 1, 2016
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