MCCs & Distribution Panels

IEC 60529 specifies protection ratings for enclosures. These ratings describe the level of protection against dust and liquids entering the enclosure.

IP ratings consist of two numbers. The first number describes the protection against solid objects and the second number describes the level of protection against entry of liquids.

NEMA 250 is a product standard that addresses many aspects of enclosure design and performance. 

*NEMA and IP ratings are not directly equivalent and this information provides an approximate correlation only.

• compartment types
• method of access to compartments
• safety levels provided during access
• effect on continuation of service during access
• type of insulation barriers between compartments
• internal arc endurance (refer to section on internal arc classification)

The manufacturer must state which areas of the switchgear are accessible and provide a clearly defined switchgear classification

Switchgear is rated according to IEC 62271-1. When choosing switchgear, its rating must be sufficient for the electrical characteristics at the point of installation, the environmental conditions it needs to operate under, and the safety requirements. Future expansion of the switchgear distribution system needs to be considered, as this may affect initial rating requirements.

Switchgear selection is determined by considerations including:

Electrical conditions

• System operating voltage (U)
• System operating frequency (f)
• Nominal operating current (I)
• Short circuit current levels at the point of installation (I_SC, I_dyn, etc)
• Horizontal busbar arrangement

Environmental conditions

• Ambient temperature
• Altitude
• Pollution degree
• Indoor or outdoor installation

Personnel safety considerations

• Internal Arc Classification (IAC)
• Interlocking of access areas and switchgear apparatus
• Access method (e.g. tools, keys, process, etc.)
• Withdrawable switchgear apparatus

ANSI defined switchgear is equivalent to IEC classification LSC2B-PM, with the following characteristics:

• the main switching device is withdrawable, with disconnecting auxiliary control circuits
• separate compartments are provided for voltage transformers and control transformers
• busbar compartments are divided between adjacent enclosures
• metal barriers isolate the withdrawable compartment, when the main switching device is drawn-out into test position
• main circuit busbars and connections are covered with fire resistant insulating material
• mechanical interlocking prevents the withdrawable switching device from being moved into service position
• low voltage control parts are segregated from medium voltage apparatus
• all voltage transformers must have primary circuit current limiting fuses

Interlocking between different switchgear apparatus and enclosure access covers and doors enhances personnel
safety, as well as improving operational convenience. If a switching device can cause serious damage in an incorrect
position, this must also have a locking facility.
Interlocking uses electrical and mechanical methods or a combination of both.
IEC 62271-200 states mandatory rules for switchgear interlocking:

For metal-enclosed switchgear with removable switching apparatus:

• the switching device must be in the open position before it can be withdrawn
• the switching device can only be operated in the positive service or test position
• the switching device cannot be closed unless the auxiliary control circuits required to open the switch are connected. Auxiliary control circuits cannot be disconnected with the switching device closed in the
  service position

For metal-enclosed switchgear with disconnectors:

• a disconnector cannot be operated under conditions other than those for which it is intended to be used
• a disconnector cannot be operated unless the main switching device is open
• operation of a main switching device is prevented unless its associated disconnector is in a positive service, test or earth position
• disconnectors providing isolation for maintenance and servicing must have a locking facility

Metal enclosed switchgear can suffer internal faults at numerous locations, causing a wide range of physical damage.

Internal Arc Classification (IAC) of metal enclosed switchgear considers the damage that can affect covers, doors, inspection windows, ventilation openings etc, as a result of overpressure within panel compartments. IAC also takes into consideration damage from thermal effects, ejected hot gases and molten particles.

When selecting metal enclosed switchgear, the probability of internal arcing and the safety risk to operators and the
general public needs to be considered. Where the safety risk is considered relevant, the switchgear should be IAC classified. The IAC classification indicates the maximum fault current level and duration to which the switchgear has been tested. When choosing switchgear, the IAC rating should exceed the expected fault current level and duration at the point of installation.

The rating also takes into account the accessibility of the switchgear. IAC tested and certified switchgear must always be clearly marked with the classification, fault level and duration, and accessibility of each side.

The primary standard for internal arc classification of medium voltage metal enclosed switchgear is IEC 62271-200;
IEC 62271-202 is also relevant.

IEC 62271-200 details test procedures to assess damage to switchgear from internal arcing. Test results provide the
switchgear with an IAC classification. Switchgear which passes indoor testing is also considered suitable for outdoor
use with the same accessibility requirements.

Accessibility is divided into two categories, Type A is for authorised personnel dressed with adequate protective equipment and Type B for general public access. Equipment is also tested for different directions of access: front,
rear or lateral (side).

Cotton cloth indicator panels are placed 2 m above ground level and on each accessible side of the equipment under test. If pressure relief ducts are part of the switchgear design, these must also be subjected to cloth indicator panel testing.

Tests are carried out by supplying a predetermined level of fault current for a specific duration. Using various test
procedures, the applied fault current creates an internal arc to ground within a specific region of the switchgear. In
general, test results are considered acceptable if:

• correctly secured doors and covers do not open - deformation is acceptable, providing it doesn't protrude as far as the indicator panels
• no fragmentation of the enclosure occurs within the test time - small particles up to 60 g are acceptable
• arcing does not cause any holes in the accessible areas, up to a height of 2 metres
• indicator panels do not ignite due to hot gas emissions
• the enclosure remains connected to its earth point (verified by a continuity test)

• foreign matter in the enclosure (eg vermin, metal swarf, tools)
• contamination and general degradation of insulation material
• inadequate insulation of cable terminations
• overheating of termination points due to inadequate preparation and tightening
• system overvoltage
• incorrect protection settings and coordination
  
  

Certain design techniques are used to provide a high level of safety to personnel, by minimising the effects of internal
arcing:

• compartmenting of enclosure
• pressure relief methods
• double skin panels
• arc venting away from access areas
• remote control of switchgear
• rapid fault clearance

Rapid fault clearance requires fast detection and isolation of the arc. This can be achieved using:

• light, heat or pressure sensors combined with a relay to trip a fast acting circuit breaker
• pressure operated earth switch capable of diverting the internal arc to ground (arc eliminator)
• fast acting, current limiting line supply fuses