MCB vv RCD

[NoSparks]

I was called to a customer today where the oven had blownup over the weekend. Well to be more precise the oven element had fractured and the moisture in the mineral had caused the element to pass current to earth. Now my understanding is that the RCD should have tripped out (which it did) within 20mSec and within 30mA (the measured installation characteristics). So why did the 32A MCB also trip?

All I can think of is that the current rose to 64Amps within the 20mSec. But if that is the case then shorly the whole point of an RCD preventing personal indury by limiting the maximum current to 50mA becomes nul and void.

Any comments?

 

[SirKit Breaker]

You may be missing something here. The 30mA RCD should trip within 40mS at 5 x In, Reg 415.1.1.

At 1 xIn it should trip within 400mS if my memory serves me right, but by the by, the reason it has to trip in this time is to keep the voltage below safe touch voltage. The current is 30mA because this can cause chest contractions strong enough to stop you breathing, but you can hold your breath for more than 0.4 seconds, hence the 0.4 second disconnection time.

Hope this helps,

Cheers..........Howard

 

[SPARTYKUS]

Howard thats really interesting! Thanks.

 

[MarkieSparkie]

A couple of common misapprehensions have been reiterated above:
1) When using an RCD as the protective device, the earth fault current is "automatically" limited to the rated residual current of the RCD.
2) In a TN system if Z[SUB]s[/SUB].I[SUB]Δn[/SUB]<=50V the touch voltages attained during an earth fault will not exceed 50V.
These statements are NOT TRUE!
[FONT=&amp]A drawing the earth fault path for a TN system reveals[/FONT]:
[FONT=&amp] 1) The type of the protective device has no influence on the magnitude of the earth fault current. The earth fault current is determined solely by the voltage to earth and the circuit impedances. In the "direct contact" mode, the current through the body is determined solely by the nominal supply voltage (in the 3 phase case, the nominal line to line voltage) applied across the body impedance.
2) The touch voltage hand to hand (exposed conductive part to extraneous conductive part) is given by the product of the fault current and the circuit protective conductor and main bonding conductor impedances and as neither of these is influenced by the type of protective device it follows that the touch voltage is again solely determined by the voltage to earth and the relevant impedances.

[/FONT]

 

[needasparks]

The 0.4 of a second was introduced as it was within one heart beat.
My only bit of trivia i remember from the guy who put it in the regs book.

 

[Deleted member 26818]

Our frequency is 50Hz, which means the AC current swings from positive to negative 50 times every second.
Maximum current is reached (either + or -) 100 times every second.
Divide one second by 100 and you obtain a figure of 0.01 or 10ms (this is also the time (t) to be used when conducting the adiabatic equation for instantaneous operation).
As you can see, full current would be achieved four times before a 40ms RCD would operate.
If the current is great enough, it is possible for the MCB to operate, before the RCD has a chance to operate.

 

[SirKit Breaker]

I am glad some people can explain the theory better than i can.

Cheers...............Howard

 

[nickblake]

And its also a reason why you should always work to the max Zs readings and not rely on an RCD

 

[Darkwood]

Yes Spin's nailed it, its highly likely it was a short circuit fault tripping the mcb instantly this sometimes can leave the rcd scratching its head in the on position but occasionally they trip together leaving the sparkie scratching his head.