IEC 60255-149 Measuring Relays and Protection Equipment - Part 149: Functional Requirements for Thermal Electric Relays

The IEC 60255-149 standard published by the International Electrotechnical Commission (IEC) describes the minimum requirements for thermal protection relays. This standard includes the specification of the protection function, measurement specifications and test methodologies. The purpose of this standard is to establish a common and repeatable reference for evaluating dependent time relays that protect equipment from thermal damage by measuring alternating current. Supplementary input energy quantities such as ambient, coolant, top oil and winding temperature may apply to the thermal protection specification specified in this standard. This standard covers protection relays based on a thermal model with memory function.

The thermal protection functions covered by this standard are as follows:

• Thermal overload protection
• Rotor thermal overload protection
• Stator thermal overload protection

General requirements for measuring relays and protection equipment are contained in the standard IEC 60255-1.

For the purposes of this standard,

• The expression hot curve refers to the characteristic curve representing the relationship between the specified operating time and current, which takes into account the thermal effect of a given steady-state load current before overload occurs, for a thermal electric relay with a total memory function. The hot curve is a graph of a given time-current solution for a first-order thermal system differential equation, assuming a given constant overload current and a given bias current.
• The expression cold curve refers to the characteristic curve for a thermal electric relay that represents the relationship between the relay and the specified operating time and current under reference and steady state conditions where no-load current flows before overload occurs. The cold curve is a graph of a given time-current solution for a first-order thermal system differential equation, assuming a certain constant overload current in the absence of preload.

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