Undercurrent (37); Thermal Overload (49) - Siemens 7SR18 Solkor Manual

7SR18 Applications Guide
2.5

Undercurrent (37)

Undercurrent elements are used in control logic schemes such as Auto-Changeover Schemes, Auto-Switching
Interlock and Loss of Load. They are used to indicate that current has ceased to flow or that a low load situation
exists. For this reason simple Definite Time Lag (DTL) elements may be used.
For example, once it has been determined that fault current has been broken – the CB is open and no current
flows – an auto-isolation sequence may safely be initiated.
2.6

Thermal Overload (49)

The element uses measured 3-phase current to estimate the real-time Thermal State, θ, of cables or
transformers. The Thermal State is based on both past and present current levels.
θ = 0% for unheated equipment, and θ = 100% for maximum thermal withstand of equipment or the Trip
threshold.
For given current level, the Thermal State will ramp up over time until Thermal Equilibrium is reached when
Heating Effects of Current = Thermal Losses.
The heating / cooling curve is primarily dependent upon the Thermal Time Constant. This must be matched
against that quoted for the item of plant being protected. Similarly the current tripping threshold,
the thermal withstand of the plant.
Thermal Overload is a slow acting protection, detecting faults or system conditions too small to pick-up fast acting
protections such as Phase Over-current. An Alarm is provided for θ at or above a set % of capacity to indicate
that a potential trip condition exists and that the system should be scrutinised for abnormalities.
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Figure 2.6-1 Thermal Overload Heating and Cooling Characteristic
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