Amps and milliamps

[Tom Evans]

I was just pondering.....if a consumer unit has say a 32mA mcb protected circuit, upon which I use an appliance with a 15 Amp fuse in it's plug........ wouldn't the mcb always blow before the 15 amp fuse in the plug ????? 15 AMPS seems to be so much tougher than 32 mAmps. Or don't I understand amps and milliamps ? Can somebody please tell me and put me out of my misery.... or shoot me

 

[SirKit Breaker]

You are way off the mark.
The final circuit will be protected for overload by a MCB (miniature circuit breaker) this will be rated depending on the circuit it is protecting so say its a radial socket circuit wired in 2.5mm twin and earth cable it will have either a 16amp or 20 amp mcb.
The circuit may also instead of a MCB have a RCBO (residual current circuit breaker with overload) this will be again rated at 16amps, but it will have 30mA earth leakage protection.
Some Consumer units have RCD (residual current device) to protect several MCB's as it is a lot cheaper than using RCBO's

So, what you need to understand is the MCB protects the circuit by not letting it get overloaded, and will trip off in less than 0.4 seconds.
The RCD or the RCBO protects the user (you) and also does the job of the MCB by disconnecting very quickly usually around 15 milliseconds once it senses earth leakage 30mA. It is 30mA because this is not high enough to kill, but is high enough to stop you breathing, but as you can almost certainly hold you breath for 15 or 20 milliseconds this is deemed a safe leakage current to disconnect at.

Hope that explains it a bit for you.

Cheers.......Howard

 

[SimpleSimon]

It's a 32AMP circuit breaker. You are getting this confused with a 30 MILLIAMP RCD which is smaller. But will not trip because it is there to detect an imbalance between the live and neutral by monitoring the amount of earth leakage current, if 30mA is exceeded (you may find it could less than this) than the device will trip.
A 32AMP MCB will trip when the circuit it's protecting is either overloaded (could take a while) or if a short circuit live to neutral/earth, neutral to earth happens. (instanteneus).

 

[Tom Evans]

Thanks both. So let me get this straight.
MCBs are in amps and RCDs are in milliamps.
So MCBs can carry several amps but a RCD will trip while detecting a tiny milliamp imbalance ???
Have I got it right ? LOL

 

[Engineer54]

Sirkit,

''It is 30mA because this is not high enough to kill, but is high enough to stop you breathing, but as you can almost certainly hold you breath for 15 or 20 milliseconds this is deemed a safe leakage current to disconnect at.

This a common misconception, as in the right circumstances and conditions as little as 6 to 10mA can most certainly kill... 30mA is basically on the threshold of heart fibrillation, and therefore considered as safe in ''most'' situations. It's a compromise in truth, between a stable safe person protection level and nuisance tripping...

 

[SimpleSimon]

Thanks both. So let me get this straight.
MCBs are in amps and RCDs are in milliamps.
So MCBs can carry several amps but a RCD will trip while detecting a tiny milliamp imbalance ???
Have I got it right ? LOL

Yes because their functions are both different.

 

[Tom Evans]

By George, I think I've got it

 

[timo]

RCD's are also rated in AMPS - Eg, 63A 30Ma or 80A 30Ma etc also 100Ma rated RCD's

 

[Hawk81]

RCD's are also rated in AMPS - Eg, 63A 30Ma or 80A 30Ma etc also 100Ma rated RCD's

While this is all true you need to understand that an RCD does not offer overload protection. The amps rating on an RCD is just a safe working load rating.

 

[telectrix]

it's all been said already, so i have nothing to add. except an obvious one-- 1mA is one-thousandth of an amp, or 1mA = 0.001A.

 

[DavidJ91]

So what you're saying is:

A 30AMP MCB (example) protects a particular circuit from faults, whereas a 30MILLIAMP RCD protects a circuit/multiple circuits via MAINS power as that's where earth-leakage occurs??

*Waits for someone to* ----> <----- me

 

[Thenitwentbang]

So what you're saying is:A 30AMP MCB (example) protects a particular circuit from faults, whereas a 30MILLIAMP RCD protects a circuit/multiple circuits via MAINS power as that's where earth-leakage occurs??*Waits for someone to* ---->

They protect for different things in different ways.

The MCB is a thermal device. It uses the thermal effect of the current to sense an over-current situation and disconnects if this happens. Imagine a ring, protected by a 32 Amp MCB, which you then proceed to load with a kettle at every socket outlet - all turned on at once. The excessive current demand causes the MCB to heat up and trip (in .04 Seconds) before the circuit disconnects itself by burning your house down.

The RCD is an Imbalance Device. It looks for an imbalance between normally live conductors (line and neutral) and disconnects the circuit if this condition is met. Imagine a line conductor has come adrift inside your cooker and touches the earthed case. There is now an imbalance between the current flowing out through line and back through neutral (because the return path is now through the CPC). In this case the imbalance causes the RCD to disconnect the supply - in just a few Milliseconds and before it is likely to do you any permanent harm.

 

[Thenitwentbang]

It's worth pointing out, also, that the MCB would also be tripped by a fault condition. A direct short circuit will cause excessive current to flow as will an earth fault condition.

 

[DavidJ91]

...The RCD is an Imbalance Device. It looks for an imbalance between normally live conductors (line and neutral) and disconnects the circuit if this condition is met. Imagine a line conductor has come adrift inside your cooker and touches the earthed case. There is now an imbalance between the current flowing out through line and back through neutral (because the return path is now through the CPC). In this case the imbalance causes the RCD to disconnect the supply - in just a few Milliseconds and before it is likely to do you any permanent harm.

So you're saying and RCD is rated at such a small amount (30mA) because it means it'll trip much quicker than a normal MCB; and that the imbalance would be when there's current in the live, but no residual current flowing through the neutral, as it's going through earth instead(?)

Thanks so much for clearing that up for me/us trainees

I shall now favourite this thread for future reference

 

[Thenitwentbang]

So you're saying and RCD is rated at such a small amount (30mA) because it means it'll trip much quicker than a normal MCB; and that the imbalance would be when there's current in the live, but no residual current flowing through the neutral, as it's going through earth instead(?)Thanks so much for clearing that up for me/us trainees I shall now favourite this thread for future reference

Essentially, that's correct. Under fault conditions, some of the current in the line will not use the neutral as the return path. When that happens, it creates an imbalance and when the imbalance reaches the set level (in this example, 30 ma - or 30/1000ths of an amp) the device automatically operates.

Although, as you say, the RCD will operate more quickly, you would not want to use it as your only protective device; an RCD will not automatically protect against an over-current situation, where there is no earth fault. The reason for that is that overloading a circuit does not, by itself, cause an imbalance - it simply flows too much current through the line / neutral path. Hence the need for MCBs, also - or RCBOs, which combine the functions of both RCD and MCB.There's some good info on RCDs in the OSG - starting at page 91.