ring main

[snowplough]

hI guys,

I am new on here so i have loads of questions to ask you all as i am at college in my second year doing my 2330 .

First question, why is a (30amp rewirable, 32 amp mcb) circuit breaker on a ring main used for protection as it is covering a 27amp cable , which i thought should be equal to or greater then the operating current. Surel the cable could melt before the fuse wire or cb would trip?.

many thanks for reading this

Snowplough

 

[sparks1234]

Look into the title of your thread, you have already told us it is a ring main, so you dont have just one 2-5 cable feeding your sockets, you also have one coming from the opposite direction so effectively giving you 5mmCSA of copper

 

[Notsosmart]

The idea of a ring circuit is that you are effectively doubling up on the cable size but saving on copper. So depending on installation ref method & correction factors etc then 27amp rated cable (table 4D5) theoretically has a current capacity of 54amps (then apply correction factors etc)

 

[snowplough]

Many thanks for that sparks 1234, notsosmart,

Second question,

When you do an R1and R2 test on a light circuit say, it says to go to the furthest point in the circuit, which i asume would be the last ceiling rose in that circuit or would it be the switchthat feeds that rose?.

Secondly once you have taken your R1 AND r2 reading , and then say you check the continuity of the cpc at each switch by putting your probes on a live conduct and the cpc in every switch, what about the strappers in the two way and intermidiate ,im thinking if you say have astrapper that is not terminated prop or is faulty and has really high resistance then if not checked when doing continuity test then surely this would be weakest link in the circuit and the true R1 and R2 reading.

The reason im confused is when im doing the cont test at college we are told to just find a live in each switch and put the other probe on the earth, but as i said above surely this isnt testing all the strappers.

Obviously, this is the method where you loop the line and earth at the consumer unit.

Sorry if it doesnt make sense what im tryiing to ask but i am confused

Many thanks

Snow plough