wireless fuses/MCBs/RCB's/RCD's?

[SparkyTris]

Could someone give a definitive run-down of all the similar-sounding 3-letter acronyms that apply to these modern fuse units in the UK? By this, I mean the modular switch components that we now use instead of wire fuses.

Also, how do they work? I had an idea that they sensed current flowing in an earth wire, but today's experiments have proved differently.

 

[securespark]

MCB's are Miniature Circuit Breakers. They replace fuses, and give protection against short circuit and overload.

RCD's are Residual Current Devices. They give protection against earth faults. Basically, they check to see that what is sent out down the phase conductor returns via the neutral. If it does, all is well, and the RCD stays as it is. If not, there is an imbalance, and the RCD will trip, as it senses flow to earth.

RCBO's (an amalgamation of "Residual Current Operated Circuit Breakers with integral Over-current Protection") are a combination of the above two devices.

 

[ban-all-sheds]

Also, how do they work? I had an idea that they sensed current flowing in an earth wire, but today's experiments have proved differently.

What experiments?

 

[Paul_C]

Also, how do they work? I had an idea that they sensed current flowing in an earth wire, but today's experiments have proved differently.

Echo the question on what experiments. Sounds interesting......

The RCD doesn't specifically detect current flowing in an earthing conductos -- In fact it doesn't have a connection to earth at all.

What it does is to compare the current flowing in the line and neutral conductors. When the difference in current exceeds a certain amount (e.g. 30mA) it trips.

In the case of an earth fault, that imbalance will be caused by the current flowing to earth, therefore making the line current greater than the neutral. So you could say that in this case the RCD detects the earth current indirectly. But it's still possible to trip an RCD due to imbalance even if there is no earth fault current.

 

[Paul_C]

Just to add to your list of acronyms:

RCCB = Residual Current Circuit Breaker, the former name for an RCD.

ELCB = Earth-Leakage Circuit Breaker, which came in two types, voltage- and current-operated. The latter is what became the RCCB, then RCD.

If you want to add to the alphabet soup even more, in North America the GFI or GFCI -- Ground Fault (Circuit) Interrupter -- is the equivalent of the RCD, though usually more sensitive.

 

[securespark]

Let's not confuse people more than we have to!!

 

[SparkyTris]

Experiment was this:

60W light-bulb (our handy bench-top load) wired between live and earth (not neutral) of a kwik-test gadget. Switched it on, expecting our workshop socket circuit to trip. But it didn't trip. The bulb lit up quite happily. and stayed lit.

so does that indicate a wiring fault?

our supply: comes up from ground into a wire fuse box. we take power off the top of the fuses through a heavy cable onto a modern fusebox (not sure which type of circuitbreakers other than small, and flip-switches) which controls the lights and ringmain in our small workshop. Earth line is connected to neutral at the point where supply enters building. Normally we see about 0.3V AC between earth and neutral in the workshop.

 

[ban-all-sheds]

Experiment was this:

60W light-bulb (our handy bench-top load) wired between live and earth (not neutral) of a kwik-test gadget. Switched it on, expecting our workshop socket circuit to trip. But it didn't trip. The bulb lit up quite happily. and stayed lit.

so does that indicate a wiring fault?

Are the sockets on an RCD? What rating?

 

[Canny]

From my uderstanding, that experiment should have tripped any RCD, not because of current flowing through the earth wire (at least not directly) but because of a lack of current returning along Neutral unless the RCD is not sensitive enough to trip with a 0.25A difference.

If I have my theory wrong here, please educate me in this matter. I believe most household RCDs have a 30mA sensitivity, thats 0.03A right?

 

[mapj1]

Canny, your understanding is correct - this way the trip operates even if the fault current doesn't come down the earth wire, but is through wet feet into the lawn or whatever. 30mA is the value appropriate to protect people in the UK. In some continental circuits 10ma is used for bathroom sockets, and larger tripping values are used to protect installations, but not personnel.

However, his installation may well not have an RCD at all, but only an MCB or fuse, so then you would not expect anything to trip. (If there is an RCD, either its faulty, or protects a different circuit)

Actually this method (adding a test resistance between live and earth) is a good way of assesing how much 'headroom' their is in a circuit with an RCD, as an extra 5mA (say a 5 watt 47K resistor, a few pence from RS) should not trip a circuit that is not already near the limit, but 30mA (6 such resistors in parallel....) certainly should.

Regards,
M.

 

[andy]

Experiment was this:

60W light-bulb (our handy bench-top load) wired between live and earth (not neutral) of a kwik-test gadget. Switched it on, expecting our workshop socket circuit to trip. But it didn't trip. The bulb lit up quite happily. and stayed lit.

did you connect the live before of after the bulb to the case? if after the bulb, the case will act as the neutral, and the same current will flow, meaning the MCB WILL NOT trip. the bulb would also light. if before the bulb, then it should go bang and trip the MCB. in both cases, if it was RCD protected it would trip the RCD

 

[ban-all-sheds]

Err - what case?

He describes a lamp connected between Live & Earth, my guess is via something like this:

No mention of a case, and I can't see how a lamp will make an MCB trip no matter where you connect it...

 

[andy]

Err - what case?

He describes a lamp connected between Live & Earth, my guess is via something like this:

No mention of a case, and I can't see how a lamp will make an MCB trip no matter where you connect it...

i mis-understood what he ment. as for that case, aslong as the live goes thru the bulb, the MCB wont trip, since there is still no overload, just the neutral going thru earth instead

 

[SparkyTris]

So the result of that test is that our socket circuit is protected only by an MCB.

Is it possible for the layman to tell which is which? our array of "wireless fuses" have a lot of things printed on them, but nowhere do they state MCB or RCD.

ban-all-sheds was quite correct, my kwik-test gadget was one of those things.

Under what circumstances should you use RCDs instead of MCB's? What is the difference in price?

 

[ban-all-sheds]

Easiest way to tell is that RCDs have a test button, MCBs do not.

And you use them as well as, not instead of, MCBs - the two do different things, although you can get combined devices which are called RCBOs.

Info on RCDs:

http://www.westernautomation.com/pages/demystify.htm
http://www.simonstutorials.com/downloads/rcdpresentation.pdf
http://www.memonline.com/guide06.html