G59 Mains Protection Relays – ComAp

Major updates to the G59 regulation have been annouced, effective from 1st February 2018. Learn more here. 


G59 mains protection relays ensure the safe operation of generators and the grid. They isolate renewable and local power generation facilities such as solar, wind, hydro-electric and CHP sites from grid fluctuations.

The ComAp MainsPro and InteliPro provide the fastest, most dependable and compatible route to G59 mains protection

The ComAp MainsPro and InteliPro provide the fastest, most dependable and compatible route to G59 mains protection

The need for G59 mains protection

The drive to use environmentally-friendly electricity has changed the power generation landscape. Instead of relying on a relatively small number of high-capacity power stations, we are turning towards a much larger number of low-capacity generators. Wind farms, photo-voltaic (PV) solar sites and hydro-electric plants are replacing coal and oil power stations.

Mains protection is essential when generating ‘parallel-to-mains’

The rapid expansion of lower-capacity generators can cause the grid to become less stable and this risks damaging both the generator and the grid. Systems that run ‘parallel-to-mains’ must include a relay to decouple the generator from the grid if a problem arises with either.


This 2-minute video summarises the main facts about G59.

The standard that governs mains protection in the UK and Ireland is G59. G59 mains protection is an important regulatory standard for both network operators (DNOs) and power generators. For the DNOs and utility companies G59:

  • Ensures safe balanced operation of the distribution system.
  • Guarantees reliable delivery of high quality electricity.
  • Allows smooth involvement of the distributed generation.
  • Enables remote control of power supply sources.

For generating companies, G59 offers:

  • reliable protection of their plant.
  • simple and standardised implementation across the whole of the UK.

IPU supply the most advanced and popular products in the market: ComAp’s MainsPro and InteliPro.

Panel-builders, consultants, contractors as well as distribution network operators (DNOs) all trust and know the MainsPro and InteliPro. There are 3,000 installations of the MainsPro in the UK (and over 10,000 around the world), making it one of the most popular.


What do G59 relays do?

A G59 mains protection relay continually monitors the quality and stability of the grid. It is programmed to react according to parameters set by the DNO including voltage, frequency, and ROCOF (rate of change of frequency). The standard has evolved through G59/2 and G59/3, and so parameters have been refined to improve the level of protection.

When the mains protection relay detects an event outside the programmed limits it trips a circuit breaker and decouples its generator from the grid.

The mains protection relay can also shut the generator down after an event. This is important if a generator (such as a wind turbine or solar panel farm) continues to supply power to its ‘island’ after the connection to the grid has been broken. This can prevent serious incidents. An engineer may arrive to repair the broken connection and, knowing there is no connection to the grid, start work on what he thinks is a dead cable. Without the mains protection relay, the cable could still be live.