20
5.1.1 GIS equipment > 52 kV
21
Evidence of the internal arc withstand ability of the enclosure against bursting and burn-through shall be 22
demonstrated by the manufacturer when required by the user. The IEC 62271-203 standard allows this ability to be 23
demonstrated by test or by calculations based on test results performed on a similar arrangement or by a 1
combination of both. Procedures and applications are described in [Trinh1992; CIGRE WG 23.03]. Tests must be 2
carried out with the normal insulating gas, usually SF6, at rated filling density. The switchgear is considered
3
adequate if no external effect other than the operation of pressure relief devices occurs within the specified time 4
and if escaping gases are directed so as to minimize the hazard to personnel. 5
In test practice, because equipment of this voltage class is normally SF6-filled, and release of (contaminated) SF6
6
into the environment may not be acceptable, it is common practice that such tests are performed on GIS (sections) 7
that are contained in a pressure-resistant container of adequate size. 8
5.1.2 Metal/insulation enclosed switchgear ≤ 52 kV
9
Internal arc testing of metal-enclosed switchgear is intended to offer a tested level of protection to persons in the 10
immediate vicinity of the switchgear in the event of an internal arc. Effects from an internal fault arc, such as 11
overpressure acting on covers, doors, inspection windows etc., as well as the thermal effects of arc(s), arc roots, 12
ejected gas(es) and glowing particles are included. In contrast to internal arcing in GIS > 52 kV, the standards IEC 13
62271-200, IEC 62271-201, and IEEE C37.20.7, allow no possibility of verifying internal arc withstand ability by 14
calculation, even when based on the testing of equivalent designs. For this reason, and because of the generally 15
closer proximity of medium voltage installations to the public compared to high-voltage installations, internal arc 16
testing of metal-enclosed medium voltage switchgear is very common. 17
5.2 Standardisation and Test Experience
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5.2.1 Standardisation status
19
With the release of IEC 62271-200 in 2003 an Internal Arc Classification (IAC) was defined, taking into account 20
various levels of accessibility of the switchgear: 21
• Type A: Accessible by authorised personnel only; 22
• Type B: Accessible by general public; 23
• Type C: Not accessible, i.e. out of reach (pole-mounted switchgear); 24
For assessment of the thermal effects of the hot gases expelled from the installation due to pressure rise caused 25
by the fault arc, special black cotton cloth indicators of size 15x15 cm are used in a steel frame to avoid mutual 26
ignition (see Figure 5-1). The indicators are mounted on a rack (vertically and horizontally) arranged in a 27
checkerboard pattern, covering 40-50 % of the area of the accessible switchgear side, or of the 3x3 m2 area below 28
the switchgear in case of pole-mounted apparatus. 29
An important (by test-experience the most critical) criterion to pass internal arc tests besides the mechanical 30
withstand of the enclosure is the absence of ignition of any indicators by hot gases. Ignition by glowing particles, 31
however, is allowed. High-speed video is normally used in order to make a distinction between the causes of 32
ignition. However, in many cases, the real reason of ignition (hot gases or particles) cannot be identified, for 33
example if the flight of a particle is not in the area of sight of the camera. 34
expressed in their specific weight (150 g/m2 for type A, 40 g/m2 for type B, C). The variation fabric imitates the 1
clothing of people, with the heavier cotton for the authorised personnel and the lighter for the general public's 2
garments. 3
Indicators have to be located at all accessible vertical sides of the switchgear, for type A at 30 cm distance and for 4
type B at 10 cm distance. In addition, horizontal indicators have to be installed in a prescribed way. 5
In order to represent the flow of expelled hot gases, the room in which the switchgear is to be installed is simulated 6
with a floor, ceiling and two walls perpendicular to each other (see Figure 5-1). The simulated room does not allow 7
the impact of pressure rise on the building structure to be assessed, but is only intended to represent a realistic 8
environment for the flow of exhaust gases around the switchgear. 9
Acceptance criteria to qualify the switchgear for an IAC classification are the following: 10
• Criterion 1: Doors and covers must not open. Deformations must not touch the indicator racks or walls; 11
• Criterion 2: No parts above 60 g must be projected; enclosure must remain intact during arcing; 12
• Criterion 3: Arc must not burn-through an accessible side lower than 2 m high; 13
• Criterion 4: Indicators must not ignite due to the effect of hot gases; 14
• Criterion 5: The connection of the enclosure with the earthing point remains intact. 15
Although it is easy to measure and it’s the most important factor, IEC / IEEE standards do not request a pressure 16
measurement in the arc compartment during testing. Another point to note is that, due to the different and 17
sometimes conflicting conditions in testing ( e.g. in temperature rise tests ventilation openings are welcome and in 18
internal arc testing ventilation openings are not welcome), a proper identification of the equipment and relief areas 19
of the equipment which was tested should be included in the test reports, in the absence of guidelines from the 20
main technical standards.. 21
5.2.2 Test result statistics
22
Results of internal arc tests have been analysed by a major test laboratory. Most recent statistics are based on 91 23
tests carried out in 2005 and 2006. It is the test laboratory's experience that in approximately 80 % of the tests all 24
criteria have been fulfilled, see Figure 5-2. The most prominent failure mode is related to criterion 4: the absence of 25
ignition of indicators. In 15 % of all tests, indicators ignited. 26
By comparison, earlier data (2001-2002 when IEC 60298 was in use) are also evaluated (from 137 tests): 32 % 27
did not fulfill all criteria; also in that period, in 23 % of the tests vertical indicators ignited, and in 11 % horizontal 28
ones (IEC 60298 made a distinction between ignition of vertical and horizontal indicators). 29
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