Automatic Transfer Switches to IEC 60947-6-1

The absolute criticality of maintaining power in modern industries, businesses, transportation systems and health facilities etc has led to explosive growth in the use of Automatic Transfer Switches

The Low Voltage Directive (LVD) means that Automatic Transfer Switches that are an assembly of contactors and circuit breakers can invalidate insurance and have legal implications for the contractor if the total assembly does not meet the requirements of IEC 60947-6-1.

The absolute criticality of maintaining power in modern industries, businesses, transportation systems and health facilities (et al) has led to explosive growth in the use of Automatic Transfer Switches (ATS).

These provide transfer of power, usually to an emergency standby generating system in the event of failure of the main means of supply.

Until recently, what constituted an ATS was open to some interpretation.

In Asco's case an ATS was, and is, a proprietary device for the emergency transfer of load between alternate sources of power regardless of amperage and utilisation category.

However, to many in the electrical contracting field an ATS was a device that could be assembled relatively cheaply using contactors or circuit breakers.

This second view of what constitutes an ATS was always erroneous - an assertion now backed up by IEC standards - although the issue was contentious.

The major weakness of this approach, was the lack of understanding that the performance criteria for ATS units are far more severe than those for contactors, disconnect switches and circuit breakers.

This is for very sound engineering reasons.

The ATS must be able to survive severe anomalies and restore critical loads to service in a timely manner.

The first problem with using contactors - usually in mechanically interlocked pairs - in automatic transfer systems is that typically they are limited in amperage.

In contrast, Asco's newest ATS, the 7000 series, is rated up to 4000 amperes.

The second problem is the possibility of the contacts stopping in between, or being connected to both sources simultaneously.

In addition, if a fault condition should occur, magnetically held contactors are likely to open.

This will likely lead to destruction of the contacts or, at the very least, contact welding.

Such an occurrence will render the transfer system inoperable.

All of these shortcomings have been addressed in the IEC standard 60947-6-1 which has been implemented in Europe as part of the Low Voltage Directive.

The European Union through the issuance of Directives has imposed compliance with applicable IEC standards as a prerequisite for products to move in channels of commerce within the European Community.

As evidence of this, products must bear the CE mark.

Moreover, Automatic Transfer Switches bearing this mark must be tested to and meet the requirements of the EMC and Low-voltage directives.

However, it is not enough for products just to bear the CE mark.

It is important that they carry this marking for qualifying to the applicable standard covering the equipment for the desired application.

The manufacturer can conduct testing upon his own behalf to achieve this and then issue a certificate of conformance.

Asco undertook its own testing to this effect.

However the company also took the extra step of having an EU notified body witness its testing and product performance with regard to the IEC standards, both for the LV and EMC directives.

The result of this independent testing is that standard Asco ATSE products shipped into the European Union not only bear the CE mark but they also bear the KEUR registry mark of KEMA, the recognised notified body of The Netherlands.

This signifies their compliance, via an approved third party body, with IEC 60947-6-1.

The requirement for products to qualify to the applicable standard means that, where a product is to be used as a contactor, it should comply with UL 508 and IEC 60947-4.

Where the product is intended for use in transferring loads between two power sources, it should comply with UL 1008 and IEC 60947-6-1.

These examples are drawn for a specific reason.

Switching devices qualified to IEC 60947-4 are allowed to have contact welding as a result of some of the qualifying testing.

If this product were to be used in a load transfer application, contact welding would be a detriment to required performance.

For this reason, IEC 60947-6-1 states in Appendix A Par.

A3 " Contactors and circuit breakers - have been tested - no less severe than those specified in this standard".

Even though the device is marked showing compliance with an IEC standard, and it bears the CE mark, it is not recognised as suitable for application as an ATSE unless it passes the test requirements of IEC 60947-6-1.

The CE mark and conformance to IEC 60947-6-1 are required by the Low Voltage Directive.

The last paragraph has important implications not only for manufacturers of transfer switches but also for consulting and contracting engineers.

It means that a transfer system that is put together by an assembler does not comply with the requirements of BS EN 60947-6-1 unless it has been tested to BS EN 60947-6-1 as a complete assembly and is labelled as such.

Moreover, it is the duty of the consulting engineer and contractor to ensure this conformity when specifying and installing ATS systems.

Failure to do so can result in insurance being invalidated and engineers being held personably responsible should the assembly fail in its intended operation.

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