Voltage and frequency protections relay perform voltage monitoring for electrical networks and/or islanding, or load shedding for no priority loads. So when the electrical supply of a switchboard is performed by two transformers feeding each a half-busbar, in case of a transformer failure, it is possible to perform automatic load transfer from a half-busbar to the other.
Zero sequence over voltage relays protect three-phase networks with isolated or high impedance earthed neutral against earth faults.

Below the list of CEE products dedicated to the voltage and frequency monitoring.

	Device	Series	ANSI	Description
	NP V911	NP 900	59, 27, 59N, 59P/27P/47, 78, 81O/81U, 81R, 60, 74TC, 86	Busbar voltage & frequency protection IED
	NPU800	NP 800	27, 27P, 47, 59, 59N, 74TC, 81, 86	Voltage and Frequency Relay
	NPU800R NPU800RE	NP 800 R	27, 27P, 47, 59, 59N, 74TC, 81, 86	Voltage and Frequency Relay
	NPUH800	NP 800	59N, 86, 74TC	Residual Overvoltage Relay (Neutral Displacement Relay)
	NPUH800R	NP 800 R	59N, 86, 74TC	Residual Overvoltage Relay (Neutral Displacement Relay)
	TMS 700	PROCOM	27, 59, 59N	Digital multi-function voltage relay
	TMS 7000	PROCOM	27, 27P, 47, 59, 59N	Digital multi-function voltage relay with advanced functionalities
	HDG 7020	STATIC	81	Frequency relay
	TTB 7000	STATIC	27DC, 59DC	Measuring relay for DC applications
	TTG 7000	STATIC	27, 59, 59N	Undervoltage and overvoltage relays
	TTGB 7031	STATIC	60	Voltage balance relay