Ring Main Gear vs Ring Main Unit

A Ring Main Gear is the switchgear in a Ring Main Unit. And the Ring Main Unit is the assembly of one or more Ring Main Gear.

This might not be relevant, but I have done consulting, commissioning and maintenance on various different types and makes of RMU's (Ring Main Units). I have recently come across an Ormazabal unit, and was very disapointed in the workmanship and lack of information provided with the unit or from their website.

In general, these units are a low cost answer to (usually) ringmain supply and protection. It contains self-polarizing relays, which uses the current from the CTs (or in some cases it is a sensor) to power the relay. Tripping is also done via a circuit internal to the relay that produces a DC tripping current for the trip coil. On this specific Ormazabal unit, there are no indication of what the trip coil rating are.

The CT's (or sensors) must be matched with the protection relay, as some of the units cannot select different ratios and I have found on numerous occations that this has been mismatched which results in wrong primary current readings and incorrect grading as any setting applied will be incorrect.

Just a bit of useful info...

Ring main unit in electrical power distribution is a standard piece of switchgear comprising of switches for switching power cable rings of switches in series with fuses for the protection of distribution transformers. Ring main gear which includes all the switching devices associated with mainly power system protection. It also includes all devices associated with control, metering and regulating of electrical power system.