Static Transfer SYstem

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Before the IEC 62310

, prior to 2008, there was no product standard for the STS. The closest standard used by

manufacturers was the IEC 62040 (Uninterruptible power systems (UPS)). Indeed It is a good reference, but the

weak points are definitions and tests.

In fact in UPS standard IEC 62040 there is no definition for “Static Transfer System”, or of the following:

• Preferred and alternate source

• Normal mode of operation

• Transfer and retransfer

• Switching mode

• Synchronous and asynchronous transfer

• Sensitivity

• Protection class

Nevertheless the performances and the test procedures are clearly defined. First among all the size is defined with

the Current (I) and not as the UPS with the Apparent Power (S).

WHY THE USERS SHOULD ASK FOR STS IEC 62310 COMPLIANT?

First of all to have a high quality Static Transfer Switch built in line with its own specific standards.

Secondly to have an appropriate warranty on the product. In fact, before IEC 62310, there was no definition of test

and performances for STS.

PERFORMANCES

First of all the performances are defined by a code structured as “XX YY B TS”.

XX defines the fault currents management and it can be:

• CB, STS: with integrated breakers or fuses capable of bearing and of breaking specified short-circuit

currents, or

• PC, STS: without integrated breakers or fuses capable of withstanding specified short-circuit currents but

not intended for breaking short-circuit currents.

YY defines the Neutral management and it can be:

• 00 - Neutral not supported

• NC - Neutral common

• NS - Neutral separation by switching

• NI - Neutral separation by galvanic isolation

The galvanic isolation can, be achieved by means of NS STS and combined with an external transformer.

B defines the type of the transfer and it can be:

• B - Break before make (no sources overlap)

• M - Make before break (sources overlap during the transfer)

To avoid high transient currents between the sources the angle shift between them must be limited and settable.

The transfers between sources within these limits are defined either as Synchronous or Asynchronous. Make

Before Break transfers must be Synchronous.

TS defines the tolerable voltage drop characteristic by the load and it can be:

• T - total transient introduced to the load by the STS throughout an automatic transfer, or

(issues: part 1 March 2005, part 2 January 2006, part 3 June 2008)