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Creep Limit


Overview

Creep refers to the situation where the wheel on an traditional electro-mechanical energy meter moves even though there is no power being consumed. The WattNode meter has no wheel, but all electrical systems have some noise, which can cause small readings in the absence of any power consumption. To prevent readings due to noise, if the readings fall below the creep limit, the meter forces the real and reactive power values to zero, and stops accumulating energy. This is performed independently for each measurement phase using the following equation.

MinimumPower = frac{FullScalePower}{CreepLimit}

Any measured power or reactive power below MinimumPower is forced to zero. FullScalePower is defined as the nominal line-to-neutral VAC (see the section Specifications – Models for your meter) multiplied by the full-scale or rated CT current.

The default creep limit is set to 1500 (which sets the creep limit to 1/1500th of full-scale power). This sets the limit to 0.067% of full-scale power.

Some WattNode models with communication interfaces allow user configuration of the creep limit. In rare cases, this may be useful in electrically noisy environments, where you may see non-zero power readings when the power should be zero. In these cases, you might choose a smaller value for the creep limit, such as 1000 (0.1%) or 500 (0.2%). See the instructions below for each type of meter to determine if you can adjust the creep limit.

WattNode Modbus Meter

The creep limit may be configured in the field using the CreepLimit (1618) configuration register. See the section CreepLimit in the manual for details.

WattNode Pulse Meter

The creep limit is fixed at 1500 and cannot be adjusted.

WattNode for LonWorks Meter

The creep limit can be adjusted, but the instructions are complex, so contact CCS technical support for assistance.

ANSI C12 Information

There are ANSI C12 meter standards that have requirements related to the creep limit.

  • No Load Test: With nominal line voltage applied and zero current, the meter must accumulate no energy (see standards for details).
  • Starting Load Test: With a specified starting current, the meter must “operate continuously”.

The creep limit in the WattNode meter is essential to meeting the “No Load” requirement. For the “Starting Load”, the levels vary:

  • C12.1: test current at 1/800th of full-scale or higher, so the default WattNode limit of 1/1500th of full-scale is compatible.
  • C12.20: test current at 1/2000th of full-scale. To meet this requirement, the WattNode creep limit would need to be changed.

Notes

  • WattNode Pulse models initially used a somewhat different algorithm for creep limit detection that was more susceptible to measuring background noise as very small amounts of real energy. This was fixed in firmware version 18.