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Topic Title: Circuit Breaker vs Ring Main Unit
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Created On: 23 March 2013 02:59 PM
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 23 March 2013 02:59 PM
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What is the difference between circuit breaker and ring main unit?

Why ring main unit are still adopted in modern power system?

Thank you!!
 10 April 2013 10:07 PM
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A ring main unit may or may not contain a circuit breaker.

A ring main unit is, at a minimum, two switches. The use in a modern power system is to be able to section and isolate the network so fault work and maintenance can be carried out on these isolated sections.

In addition, a circuit breaker, or fuse-switch can be connected between the two ring switches. This CB or fuse-switch can control a spur circuit, or more usually, a local transformer.
 15 April 2013 07:02 PM
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I think you may be refering to 'line up' switchgear which is typically a series of circuit breakers (by the nature of the equipment specification) that can act as a Ring Main Unit and a standard 'ring main unit' type device which has 2 fault making switches and a single or multiple circuit breaker for connected loads.

The basic differences will bus bar amepere rating, RMU's are limited to lower ampere ratings of 630A or 1250A and fault rating/ operational flexibility.

RMU's are low end functional equipment that can be connected to provide a low end basic functional switchboard that is similar to a line up switchboard without all the functional control and flexibility that a 'line up' switchboard can offer, at a cost.

Horses for courses.....
 30 April 2013 06:20 AM
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Typically both are used for higher voltages. The Ring main unit has a circuit breaker to switch off the T from the ring while the rest of it isn't switched. You can go here or and look up the technical datasheets for each circuit breaker and compare the amps.

Now a switchgear has three circuit breakers, one is for 'T' off and others for each side. You can switch most parts of the circuit making them more versatile so you can add circuits easier without too many cable joints.

But ultimately I agree with the point you made in your original question especially in modern power systems.

Edited: 30 April 2013 at 10:32 AM by brontm3
 02 May 2013 09:11 AM
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 03 May 2013 05:20 PM
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What do you mean by Medium voltage?
There is LV (up to 1000V AC) and HV (more than that)
I guess you mean 11 kV ish?

Different manufacturers will provide commissioning and test procedures for their gear depending on its functionality.

A common set of tests would be:

Functional tests.
Triping and closing the breaker (manually/remotely and telecontrol if available)
Closing the breaker to earth.
Checking function of interlocking systems

Tripping tests via protection
Depending on the protection (say restricted earth fault if the breaker protects a transformer; or overcurrent if protecting a circuit); you inject the relays and check the functioning of the breaker to make sure it trips when it should do.

Primary injections to check the ratio of any windings in the protection system; secondary injections to ensure the protection operates at the correct current levels.

Trip timing tests - checking how long it takes the breaker to trip under fault conditions.

Telecontrol/Scada tests
Ensuring correct signals are received remotley if the breaker is connected with this function

There will be maintenance testing - ensuring integrity of isolation if it is a vacuum or gas breaker with a pressure test; continuity and insulation tests; mechanism checks; etc etc.
If it is an oil breaker there is lots of tests and checks - dielectric strength, acid content, water content of the oil and also checks on the contacts for arcing etc; gaskets etc etc. The list is long for maintenance, and maintenance manuals will be printed for each breaker. Typically takes 3 or 4 hours to to full maintenance on such a breaker. There is a lot involved.
 16 September 2013 04:39 PM
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ring main units are still used in distribution as they are the most cost effective way of running a system.

they generaly have two switches and a circuit breaker or fused switch on the "T" to protect the transformer. you would run this system on either a open ring or radial circuit and is very cheap and easy to manage. all of this could be installed outside as most RMU's can be installed outside.

the other is a closed ring system. this means you have to have a hv panel with circuit breaker for each circuit. with this you also need unit protection. it is a very good system as you can have a fault on a cable and not loose any transformers but also very expencive and you wouldnt be able to house this equipment outside you would need to build appropriate housings for it.

so there you go. pretty must the reason why we do anything these days, COST!

hope this helps


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