Heat generation estimation for historic switchgear Type D (to 4.76kV) and Type F (to 15kV) with air magnetic circuit breakers

We are often asked for estimated heat generation data for our equipment. These requests are most common for our modern products, but we also have requests for data on the types D and F switchgear designs which used air magnetic circuit breakers. This issue of Tech Topics provides information on these historic designs to allow calculation of approximate heat generated by the switchgear under assumed loading conditions.

The heat generation data in tables (pages 2-3) is based on full rated continuous current. Actual heat generation estimates must consider the true loading of the equipment. The amount of heat generated is related to the square of the current, so a circuit breaker operating at one-half rated current will have heat generation only one-quarter of that at full rated continuous current. Because the effect of the square relationship is very significant, it is overly conservative to estimate heat generation based on the assumption that all sections and all circuit breakers each carry their rated continuous current at all times. Air conditioning systems sized based upon such estimates will be larger than the real operating conditions require.

To estimate the heat generated under actual loading conditions, determine the component heat generation for each of the components indicated in the table. Estimated heat generation for circuit breakers should be adjusted for actual loading based on the ratio of the squares of the actual current loading for each individual vertical section, but this is frequently ignored in the interests of simplification. Instead, the main circuit breaker loading is usually assumed to be equal to the main bus loading in all vertical sections.

Example: Assume a lineup of type F switchgear, with five vertical sections, one 2000A main breaker (loaded to 1400A), three 1200A feeder breakers (loading 250A, 600A, and 550A), with 2000A main bus and including space heaters. The lineup includes one VT rollout, one 10kVA CPT, and non-complex electromechanical relaying (e.g., three induction disk relays) and instrumentation (e.g., an ammeter with selector switch). The calculation would be as follows:

If true loading were not considered, (i.e., all calculations performed on the basis of full rated current), the calculations would yield a heat generation of 8420 watts, or about 170% of the "real" heat generation.

Notes on assumptions:

1. Space heaters, when provided, are not normally controlled by a thermostat; hence, their load is represented as a continuous load. The purpose of space heaters is to prevent condensation, and this is not limited by the absolute temperature. Even when a thermostat is used to control the heaters, it is set to shut the heaters off at a temperature of approximately 110 degrees F. Therefore, in an air conditioned room, the heaters would be energized continuously.
2. Heat generated by current transformers is ignored as it is usually insignificant and varies according to the CT ratio as well as the loading.
3. The CPT heat generation estimate is very conservative and assumes that the CPT is operated at full rated capacity. If normal loading is at less than full rating, heat generation may be adjusted by the square of the percent loading.
4. Relaying and instrumentation heat generation estimates are very approximate, and are normally estimated on the basis of the number of circuit breaker cells. Extensive relaying and instrumentation may warrant additional conservatism in the estimation of associated heat generation.
5. Conversion factor: watts x 3.415179 = BTU / hour.

Notes on assumptions:

1. Space heaters, when provided, are not normally controlled by a thermostat; hence, their load is represented as a continuous load. The purpose of space heaters is to prevent condensation, and this is not limited by the absolute temperature. Even when a thermostat is used to control the heaters, it is set to shut the heaters off at a temperature of approximately 110 degrees F. Therefore, in an air-conditioned room, the heaters would be energized continuously.
2. Heat generated by current transformers is ignored as it is usually insignificant, and varies according to the CT ratio as well as the loading.
3. The CPT heat generation estimate is very conservative and assumes that the CPT is operated at full rated capacity. If normal loading is at less than full rating, heat generation may be adjusted by the square of the percent loading.
4. Relaying and instrumentation may warrant additional conservatism in the estimation of associated heat generation.
5. Conversion factor: watts x 3.415179 = BTU / hour.