Important aspects regarding device temperature rise;

Recommendations 6.1.4.1 Rated current

For many low-voltage components (for example circuit breakers, load switches, contactors, fuses, conductors), ohmic losses are the main sources of losses. They are proportional to the square of the operational current. The r.m.s. value is definitive. Under variable load conditions (for example intermittent operation) the r.m.s. value can be averaged over time, if the cycle time is shorter that the heating time constants of devices. In the power range up to around 40 A the permissible integration time (= cycle time) is around 15 … 20 minutes.

I1

Fig. 6.1-4

Example of calculation of the effective value for intermittent operation of a motor.

t

t1 Starting time at starting current I1

t2 Service period at operating current I2

t3 Interval time at current I3=0 t₁+ t₂+ t₃ Cycle time

As the operating conditions often deviate from those for determining the conventional thermal current in the open air I_th (see above), as a general rule of thumb it is recommended not to operate devices at over 80 % I_th. At 80 % current, the current-based heat losses (ohmic losses) are reduced to around 64 %.

6.1.4.2 Thermal protective devices

In protective devices such as circuit breakers or motor protection relays with narrow adjustment ranges, the 80 % recommendation can not always be observed as the devices must be set to the rated current of the load to be protected and often the overlap of the current ranges is insufficient. As far as possible, a current range should be selected that enables for a setting and hence operation, in the low to medium range of the scale.

For bimetallic protective devices it should be noted that the heat generated in the bimetal strips required to provide for the quality of protection is roughly the same for all current ranges of a frame size. A 1 A bimetal relay at 1 A generates approximately the same heat as a 10 A bimetal relay of the same size at 10 A.

6.1.4.3 Conductor cross sections

A substantial quantity of heat is removed from the devices via the connected conductors. The larger the cross section the better is the cooling effect.

During the manufacturer’s temperature-rise tests, attention is paid to compliance with the

temperature-rise limits as of Tab. 6.1-1, the temperature rise of internal components used within the devices and their compatibility with the materials used. At increased ambient temperature, for example when the devices are installed in cases or cabinets, larger cross sections of connecting conductors are required than those used in the type tests and those corresponding to the regular installation tables, which are based on an ambient temperature of 30 °C. In practice, selection of a conductor that is ”one size up” in cross-section is recommended. This also has the advantage that the heat dissipation in the switching cabinet and the energy consumption of the installation are reduced because of the lower current density in the conduc-tor. If necessary, two conductors can be run in parallel.

With bimetal relays and circuit breakers with bimetallic tripping mechanisms, the cross section of the connected conductor affects the ultimate tripping current. Typically, a larger wire cross section can, depending on the temperature compensation of the bimetal strips, lead in practice to an increase of the ultimate tripping current by up to 5 %. From this point of view it is advanta-geous, rather than choosing the device with the highest current range of a frame size of bimetal relay or circuit breaker, to choose the next largest frame size.

t1 t2 t3

I2 I3

I

t

LVSAM-WP001A-EN-P - April 2009

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The selection of conductors with a higher insulation class does not affect the rate of heat-flow out of the devices. For this reason, their cross-section should be the same as those of conduc-tors with a 70 °C limiting temperature.

In the case of busbars it should be noted that, for the same reasons, the load capacity of busbars that are connected to devices is lower than the load capacity of busbars that are exclusively serving for power distribution. The corresponding tables can be found in the annex to IEC 60890.

6.1.4.4 Conductor length

As shown in Section 6.1.2 in the type tests for devices comparatively long connecting lines to the terminals are used and these help to radiate a substantial amount of heat from the devices.

With short connections this does not occur. As a consequence the temperatures of the termi-nals, the device interiors and the conductors themselves rise even if the load remains un-changed. For this reason, with compact device assemblies such as for example motor starters consisting of a circuit breaker and a contactor, type tests of the complete starter including the connecting (power wiring) components are performed. The power wiring modules have a higher temperature withstand than normal wiring material and the tests ensure that the temperature-rise limits for all the components are observed.

Fig. 6.1-5

With compact motor starters with power wiring modules, the type test ensures that the limit temperatures of all components are observed.

With short connections in individually wired installations, compliance with the temperature limits should be ensured by load reduction and/or forced cooling. The selection of larger connecting cross-sections increases the heat exchange of mutually connected devices and reduces the amount of heat dissipation in the conductor itself. Therefore the rate of heat-flow to the outside is not improved.

6.1.4.5 Tightening torques

In the catalogues and on the devices themselves, often ranges for the tightening torques of the terminals are stated. These then relate to all current ranges and the respective wire sizes for a frame size. From the point of view of device heating it is a good idea to always use a value in the upper part of the torque range as this will have a positive effect on the electrical and thermal transition resistance and hence the heat generation and flow. See Fig. 6.1-6. The upper range limit should not be exceeded so that the mechanical strength of the terminals is not unaccepta-bly stressed.

6.1.4.6 Line ducting

As can be seen from the relevant tables (see also RALVET) for conductor selection, the method of installation (in the open, on tracks, in cable ducts etc.) and the accumulation of conductors have a large influence on their load-carrying capacity. The more heat-flow to the surrounding air is prevented, the lower is the load capacity or, in other words, the greater is the required cross-section for a given current. For technical reasons therefore, the lines should be laid as loosely as possible. Lines that are routed into a cable duct only a short distance from the connection

terminal have a relative short open length over which heat can be dissipated and they mutually heat each other in the duct.

6.1.4.7 Operating frequency and harmonics

All normal technical data and tests relate to the normal supply frequency of 50/60 Hz. At higher frequencies additional losses occur that adversely affect the loss balance or reduce the load capacity of the devices. See Section 2.4.3.

6.1.4.8 Mounting devices side-by-side

In real-life switchgear assemblies, the switchgear devices are very often placed in rows side-by-side. Circulation of the ambient air between the devices is then not possible and as a conse-quence the rate of cooling of devices in comparison to the standard test conditions is reduced (see Fig. 6.1-5). Where this results in an inadmissible temperature rise, then a reduction in load capacity will occur.

In practice, adjacent devices are frequently not loaded at the same time or the devices are operated with currents that are well below the conventional thermal current in open air (I_th). In such cases, adjacent placement of devices is permissible with respect to temperature-rise.

Care is required when operating adjacent devices close to the Ith and in case of a combination of adverse factors with respect to the heating as described above. In such cases, spacing between the devices is recommended in order to reduce mutual heating. Often instructions are included in the manufacturer’s information – for example with the dimensional drawings

(catalog, packaging, application recommendations). To avoid hotspots, circulation of the air in switchgear assemblies is advantageous.

6.1.4.9 Mounting position

In manufacturers documentation there are specifications with respect to the permissible mounting positions and in the case of installations differing from the normal positions the

corresponding influence on the operational parameters. With respect to heating effects it should also be remembered that heat dissipation within the devices is not evenly distributed, but is concentrated on specific components, for example, the bimetal strips of circuit breakers or motor protection relays. With a mounting position that differs from normal, the mutual effect on adjacent devices can also change.