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# MCB Tripping

Discuss MCB Tripping in the Industrial Electrician Talk area at ElectriciansForums.net

J

#### Jim Newman

Welcome to ElectriciansForums.net - The American Electrical Advice Forum

Hi, this is my first post.

I have just had 2 bathrooms renovated with lots of new lighting which is on the same 5 amp mcb as the adjacent 2 bedrooms. I have calculated that the total demand on the mcb is 3.3 amps. The consumer unit is about 18 years old and is a Wylex NNEM 1214. The 5 amp mcb would trip after 1 or 2 minutes when all the lighting is switched on so I exchanged the connection with an adjacent 5 amp mcb which trips after about 20 minutes.
I don't understand why a 5 amp mcb should trip when the demand is only 3.3 amps.

1. What do you recommend?
2. Should I replace the 5 amp mcb with a 6 amp one which I have seen on webpage: 6 Amp Wylex MCB ?
3. Would it fit physically?
4. Do mcb's get tired after years of use?

I am not an electrician but do have a degree in electronics so it is well within my capacity to make the change if you think this is the best course of action.
Thank you.

Jim Newman.

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S

#### Shakey

Hi Jim, Yes MCB's will age, as fuses do

As a basic guide, a type B MCB has an instantenous trip current 3 to 5 times its a nominal value.

So a 6A type B will trip instantly at between 18 and 30A. The charts in the regs show that for a current of 8.5A, the MCB would never blow. At about 9A you are looking at about 4,000 second trip time

As the current rises, the trip time reduces

If you had a 6A Type B BS60898, with a trip time of 2 mins, you had about 10.5A flowing

The values are not vastly different if you using a fuse, for example

How did you calculate the current?

When we plan lighting circuits we allow a nominal value of 100W per lamp holder

P=IV so a 6A breaker would give 6 x 230 = 1380W, or 13 lampholders

at your calculated 3.3A, you would have 759W, which is seven lampholders, which is what concerns me when you say 'lots'

Ok you could take actual lamp ratings, but that is bad practice as it does not allow for the lamps to be changed to different values

The fact that two trips give different trip times, leads me to believe that the trip is doing what it is designed to do - trip on a sustained overload.

It is unlikely you will get a 6a 60898 to fit an old board, but you could try - but that doesnt change the fact that you have probably overloaded the circuit

Hope this helps

J

#### Jim Newman

1st Bathroom, 5 x 35W 12V downlights = 175W.
1 x 40W 240V ceiling light = 40W.

2nd Bathroom, 5 x 35W 12V downlights = 175W.
2x 20W 12V ceiling lights = 40W.
(Extract fans not included, they are off most of the time).

1st Bedroom, 4 x 35W 240V downlights = 140W.
2 wall lights, each 40W = 80W.

2nd Bedroom, same as 1st bedroom = 220W.

Total 870W.

As you say, V x I = W, therefore I = 3.6 amps. This is well below the rating of the 2 mcb's that I have tried, one of which tripped after 1 or 2 minutes and the other after 20 minutes.
I fail to understand why they are tripping. I would have expected a 5 amp. mcb to allow a maximum load of about 1200W.

Thank you.

Jim Newman.

S

#### Shakey

Ok jim, on the face of it you should be nowhere near an overload sitiuation

Now I would try and identify 'patterns' to the trip

(Ideally you would test circuit current using a current clamp - i am assuming you dont have one!)

does it only happen if all lights are on? Does the problem go away if one particiular bank is switched off?Try operating light switches and see if you can identify a particular part of the circuit that causes the problem

Because you have a number of tranformers on the circuit, you may have fauly one with a 'partial' short circuit on the input or output windings - there may be sufficient left to operate the lamps, but enough of a problem to draw excess current

if you had access to an insulation tester, I would be looking to insulation test the circuit as well (after diconnecting loads as appropriate)

be patient and try and establish a pattern somewhere

C

#### Current Sniffer

Hi Jim,

Yea, Shakey is spot on you just need to find the device thats causing you the trouble.
Try turning on the lights in severl diffrent groups until you can find the set of lights that trip out. I'm sure you have done this already but the first step is to only turn on one set at a time and then diffrent groups.
Have fun!!

J

#### Jim Newman

Thank you both for your helpful replies. I tried each room one at a time and the mcb did not trip. Although there was no apparent overload situation it was behaving as if there was. Perhaps the mcb's in this 18 year old board are tired?
Anyway, I have installed a 6 amp mcb and the problem is solved. Apart from the 6 amp mcb possibly allowing an overload on what is 5 amp wiring I cannot see a disadvantage in this. I did think of just installing a new 5 amp mcb working on the theory that the existing 5 amp mcb's are 'tired'. Perhaps I should have.
Thank you for taking the trouble to help me.
Jim Newman.

S

#### Shakey

Jim, it is unlikely you have 5A wiring

The smallest twin and earth is 1mm, which is generally used for lighting circuits.

This is tabulated at 11.5A using method 1 (clipped direct)

You could even have got away with a 10A MCB.........

W

#### wayne

Jim, it is unlikely you have 5A wiring

The smallest twin and earth is 1mm, which is generally used for lighting circuits.

This is tabulated at 11.5A using method 1 (clipped direct)

You could even have got away with a 10A MCB.........

assuming no further derating factors !! stay at 6amp for 1.0mm to be sure

S

#### Shakey

assuming no further derating factors !! stay at 6amp for 1.0mm to be sure

Hi Wayne, thats why i said 'could have'

clipped direct with no derating, a 10a MCB on 1mm is perfectly acceptable and i.a.w the regs

J

#### Jim Newman

Thank you all for your replies. I am reassured that the 6 amp mcb is ok from a safety point of view.
Jim.

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