Overload relay selection

[chuck-h]

It is advisable to include a standalone overload relay in each feeder circuit managed by the Load Manager.

In the US, most jurisdictions use the National Electrical Code (NEC). NEC section 625.14 allows the potential total load to be greater than feeder capacity if a load management system prevents the actual load from exceeding capacity.

Relying on the load management software logic, in cooperation with the individual EV charger logic, to enforce the load limit can present approval difficulties (depending in part upon the strictness of the electrical inspector reviewing the project). It is difficult to get an independent test lab to "type approve" an open system whose performance depends on the cooperation of products of different manufacturers. Furthermore, it is technically difficult to ensure "fail safe" current limiting in case of intermittent communication (packet drops), loss of power at the controller, or software bug.

Misbehavior of the load management function can lead to circuit overload and cause the feeder circuit breaker to trip. Some inspectors consider this insufficient protection as circuit breakers do not have a high cycle life and should not be relied on to deal with potential "routine" faults such as communications errors.

An overload relay is rated for repetitive operation. "Smart" overload relays can include a current measurement function which reports over a communications link. This function (whether part of an overload relay or not) is desired for load management. Commercially available overload relays primarily target motor applications, but can be applied to any circuit.

See discussion of communication with overload relay as a current sensor at #3

[chuck-h]

Overload relays are often (but not always) separate from the power contactor. When separate, they are usually designed to mount directly to the same manufacturer's contactor. Overload relays are usually designed for 3-phase loads and require special configuration (see manufacturers' instructions) when used on single-phase loads, either L-L or L-N.

Allen-Bradley E300, E1+ https://ab.rockwellautomation.com/Circuit-and-Load-Protection/LV-Motor-Protectors/Electronic-Overload-Relays
Siemens https://w3.siemens.com/mcms/automation/en/industrial-communications/io-link/industrial-controls/overload-relays/pages/default.aspx
ABB/GE (no communications) https://apps.geindustrial.com/publibrary/checkout/DET-832?TNR=Brochures%7CDET-832%7CPDF&filename=DET-832.pdf
Littelfuse https://www.littelfuse.com/products/protection-relays-and-controls/protection-relays/motor-and-pump-protection/777.aspx
Eaton C440 http://www.eaton.com/ecm/idcplg?IdcService=GET_FILE&allowInterrupt=1&RevisionSelectionMethod=LatestReleased&noSaveAs=0&Rendition=Primary&dDocName=BR04210001E
Schneider Electric (no communications) https://www.schneider-electric.us/en/product-range/1885-tesys-lrd-overload-relays/

Example sources for external contactors used with overload relay modules: https://www.automationdirect.com/adc/shopping/catalog/motor_controls/iec_contactors_-z-_starters_-z-_overloads/gh15_series_contactors_-z-_starters_-z-_overloads/79mm_contactors_(80_to_110_amps)_-z-_overloads/gh15mt-3-00b
http://mfg-sol.com/store/product/3634-type-eec-3-phase-contactors