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LV switchgear functions - Isolation

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A state of isolation clearly indicated by an approved “fail-proof” indicator, or the visible separation of contacts, are both deemed to satisfy the national standards of many countries

The aim of isolation is to separate a circuit or apparatus (such as a motor, etc.) from the remainder of a system which is energized, in order that personnel may carry out work on the isolated part in perfect safety.

In principle, all circuits of an LV installation shall have means to be isolated.

In practice, in order to maintain an optimum continuity of service, it is preferred to provide a means of isolation at the origin of each circuit.

An isolating device must fulfil the following requirements:

  • All poles of a circuit, including the neutral (except where the neutral is a PEN conductor) must open[1]
  • It must be provided with a locking system in open position with a key (e.g. by means of a padlock) in order to avoid an unauthorized reclosure by inadvertence
  • It must comply with a recognized national or international standard (e.g. IEC 60947-3) concerning clearance between contacts, creepage distances, overvoltage withstand capability, etc.:

Other requirements apply:

  • Verification that the contacts of the isolating device are, in fact, open.
The verification may be:
  • Either visual, where the device is suitably designed to allow the contacts to be seen (some national standards impose this condition for an isolating device located at the origin of a LV installation supplied directly from a MV/LV transformer)
  • Or mechanical, by means of an indicator solidly welded to the operating shaft of the device. In this case the construction of the device must be such that, in the eventuality that the contacts become welded together in the closed position, the indicator cannot possibly indicate that it is in the open position
  • Leakage currents. With the isolating device open, leakage currents between the open contacts of each phase must not exceed:
  • 0.5 mA for a new device
  • 6.0 mA at the end of its useful life
  • Voltage-surge withstand capability, across open contacts. The isolating device, when open must withstand a 1.2/50 μs impulse, having a peak value of 6, 8 or 12 kV according to its service voltage, as shown in Figure H2. The device must satisfy these conditions for altitudes up to 2,000 metres. Correction factors are given in IEC 60664-1 for altitudes greater than 2,000 metres.
System voltage Impulse-withstand voltage for isolating devices kV
Overvoltage category III Overvoltage category IV
400/230V 480/277V 4 6
690/400V 6 8
1000/577V 8 12

Fig. H2Peak value of impulse voltage according to normal service voltage. Overvoltage category III and IV are defined in IEC 60664-1

  • Equipment of overvoltage category IV is for use at the origin of the installation.
  • Equipment of overvoltage category III is for all other equipments in fixed installations

Notes

  1. ^ the concurrent opening of all live conductors, while not always obligatory, is however, strongly recommended (for reasons of greater safety and facility of operation). A switched neutral pole shall break not before and shall make not after the other poles (IEC 60947-1).