Disconnector Terminal Loads: Static and Dynamic

Short answer

Each terminal of a disconnector receives the force of the conductor connected to it. IEC 62271-102 (edition 1.2, 2013) calls that force mechanical terminal load and requires two things (clause 4.103):

• The manufacturer declares the rated static terminal load. The disconnector and the earthing switch must be able to close and open while bearing that load.
• The equipment must withstand the rated dynamic load when a short circuit occurs.

The maximum static load allowed to be applied to a terminal, under the worst conditions, is precisely the rated static terminal load.

Which forces make up the load

The definition in clause 3.7.121 says the terminal load is the result of the combined mechanical forces the equipment may receive. Those forces may have different values, directions and points of application. Wind on the equipment itself is left out of the count.

With flexible conductors, the weight is already included in the longitudinal and perpendicular forces. Table 3 of the standard gives recommended static load values for two- or three-column and split-support disconnectors, as guidance only. More on the high-voltage disconnectors most installed in Latin America in Hapam, Coelme, MESA and Siemens 3DN disconnectors.

Static and dynamic

• Static load: the force the connected conductor exerts on the terminal, whether flexible or rigid (clause 3.7.121.1).
• Dynamic load: the static load combined with the electromagnetic forces under short circuit (clause 3.7.121.2).

The capability of the disconnector does not depend only on its design. It also depends on the strength of the insulators. The bending strength the insulator needs is calculated from the height of the terminal above the top of the insulator and from the additional forces acting on it.

How it is verified

The standard requires two type tests with the load applied:

1. Operation under the rated static load (clause 6.102.4). Twenty operating cycles are made with the load applied to both terminals. If the disconnector is manually operated only, 10 cycles are enough. On every operation it must close and open correctly.
2. Mechanical endurance test (clause 6.102.3). It consists of 1,000 cycles with 50 % of the rated static load in the longitudinal direction. For disconnectors with two or three insulators and horizontal isolating distance, the load is applied on both sides in opposite directions. The test is done with no voltage or current in the main circuit. The main circuit resistance is measured before and after, and it must not vary by more than 20 % from the initial value.

Class M1 and M2 disconnectors are also subjected to extended mechanical endurance tests (clause 6.102.5).

How the rated load is selected

Clause 8.102.4 asks for the rated static and dynamic loads to be selected using the requirements of 4.103 and the most unfavorable conditions the user considers. To calculate the required static load, the standard recommends starting from the specified minimum ambient temperature and one of these two combinations:

• −10 °C plus ice load plus wind load.
• −5 °C plus wind load, for tropical countries.

To calculate the static and dynamic loads, and the required strength of the insulators, add the forces of the connected conductors, including wind and, where applicable, ice on those conductors.

What not to do

• Do not use Table 3 as the rated value of a device. It is guidance; the value is declared by the manufacturer.
• Do not include wind on the disconnector itself in the terminal load.
• Do not forget the bending strength of the insulator: a terminal that holds the load is of no use if the insulator cannot bear it.
• Do not assume terminal loads apply the same way to enclosed equipment.

Mechanical strength of the installed equipment

Clause 5.103 requires that a disconnector or earthing switch with a rated static terminal load, installed according to the manufacturer’s instructions, withstand its rated static and dynamic loads without losing reliability or current-carrying capability. This requirement is verified with the tests described above, and also with the short-circuit test: the report of that test must state the static load applied to the terminals and the dimensions of the conductor used. For more detail on the reactors that limit fault current or connect the neutral to earth, read current-limiting and neutral earthing reactors.

Before and after the mechanical endurance tests, the standard requires checking operation without the load applied. One of the accepted series is five closing and opening cycles at the minimum supply voltage or pressure.

What data to gather for the specification

Clause 9.101 asks, when inquiring about or ordering a disconnector or earthing switch, to state the rated static and dynamic terminal loads and whether the conductors are rigid or flexible. To obtain those values, gather:

• The minimum ambient temperature at the site.
• Whether there will be ice, and the design wind speed.
• The conductor type, flexible or rigid, and its arrangement toward each terminal.
• The short-circuit current value that sets the dynamic load.
• The mounting conditions and whether the supporting structure is supplied with the equipment.

Related topics

The same forces on the terminals appear in measuring equipment: see the mechanical terminal loads of an instrument transformer. The disconnector shares with the earthing switch the terminal load requirement. Its drive is described in the motor operator of the disconnector. Problems at terminal connections are also detected as overheating at terminals and connections. The short-circuit current that defines the dynamic load comes from the short-circuit study.

The contact zone, which also depends on the connection, is explained in disconnector contact zone.

To see how it fits with the rest of the IEC 62271 series, see the standards for primary equipment testing.

When to call us

At Tiza Group we calculate the terminal loads of disconnectors and earthing switches with you, and verify that the bay design respects them. See our primary engineering design service. Write to us and we will send you a quote.

Related guides

• Disconnector assigned contact zone
• Disconnector classes M0, M1 and M2
• Disconnector position: locking and indication
• Earthing switch: classes E0, E1, E2
• Mechanical terminal loads of an instrument transformer
• Current-Limiting and Neutral Earthing Reactors
• Hapam, Coelme, MESA and Siemens 3DN Disconnectors

References

• IEC 62271-102, High-voltage switchgear and controlgear - Part 102: Alternating current disconnectors and earthing switches, 2001 edition with amendments 1 (2011) and 2 (2013): clauses 3.7.121, 4.103 (Table 3), 5.103, 6.102.3.1, 6.102.3.2, 6.102.4, 8.102.4 and 9.101.

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