Limitation of 63A RCD

[AndyPRK]

The way I see it is that you have a cooker and kitchen ring on the same RCD, so lets say that's 32A+32A = 64A (plus a 1A or so lighting) so that just about works if you only have a 32A cooker circuit.


The other RCD has the elec Shower (where fitted) 40A + low demand socket circuit. (say it draws 20A max) and 2A lights.

So I think 2 63A RCD's just about works if you have a gas boiler and 32A cooker circuit.

It's penny pinching, and 1 or 2 80A RCD's means you will never need to swap out an RCD in a new CU.

 

[JohnW2]

The way I see it is that you have a cooker and kitchen ring on the same RCD, so lets say that's 32A+32A = 64A (plus a 1A or so lighting) so that just about works if you only have a 32A cooker circuit.

... except that, as I said, the concept of diversity does not really exist in relation to the 'maximum breaking current' of a device such as an RCD, since that's all about the instantaneous current at the moment the circuit is broken. Given diversity, a "32A cooker circuit" could theoretically support a peak cooker load of about 83A, and many cookers (or oven+hob combinations) will have a maximum demand of 50A or more. Strictly speaking, therefore, a 63A RCD can't accommodate much more than one cooker circuit, and not much more. Similarly, a high-powered shower will 'use up' most of the current-breaking capacity of a second 63A RCD.

Kind Regards, John

 

[seasickstevie]

As it's an HMO (what we used to call bedsits) and likely every room in use at once, I'll use two 80A RCDs.

Would prefer to use a more appropriate combination of RCBOs, but when you're dealing with a landlord who counts every penny ...

 

[AndyPRK]

.. except that, as I said, the concept of diversity does not really exist in relation to the 'maximum breaking current' of a device such as an RCD, since that's all about the instantaneous current at the moment the circuit is broken. Given diversity, a "32A cooker circuit" could theoretically support a peak cooker load of about 83A, and many cookers (or oven+hob combinations) will have a maximum demand of 50A or more. Strictly speaking, therefore, a 63A RCD can't accommodate much more than one cooker circuit, and not much more. Similarly, a high-powered shower will 'use up' most of the current-breaking capacity of a second 63A RCD.

Is the current rating of the RCD about its breaking current or the amount of current its designed to carry ?

The manufactures must think differently to you, otherwise they wouldn't supply 63A RCD's in their large CU's.

 

[seasickstevie]

The manufactures must think ...

You would expect so ... but do they really?

 

[AdrianUK]

Is the current rating of the RCD about its breaking current or the amount of current its designed to carry ?

The manufactures must think differently to you, otherwise they wouldn't supply 63A RCD's in their large CU's.

But it can't really be the breaking capacity. When an earth fault hits (assuming a low earth loop impedance) the fault current is going to be many times greater than 63A or 80A. Surely the RCD MUST have a breaking capacity of the same order as an MCB??

(If the installation is PME then the phase/earth fault current must approach that of a phase/neutral fault. Its then a race between an MCB opening or the RCD opening)

 

[JohnW2]

Is the current rating of the RCD about its breaking current or the amount of current its designed to carry ? ... The manufactures must think differently to you, otherwise they wouldn't supply 63A RCD's in their large CU's.

You may be right. On looking, the technical spec for MK Sentry RCDs says:

Rated making and breaking capacity:
Normal (/m) 630A (16 - 63A) and
residual (/∆m) 800A (80 - 100A)

... although I'm not sure I understand what "Normal (/m)" and "Residual (/Δm)" mean!

Kind Regards, John

 

[JohnW2]

But it can't really be the breaking capacity. When an earth fault hits (assuming a low earth loop impedance) the fault current is going to be many times greater than 63A or 80A. Surely the RCD MUST have a breaking capacity of the same order as an MCB??

Indeed. Following from what I've just posted:

Rated conditional short-circuit current:
Normal (/nc) 10,000A and
residual (/∆m) 80A Fuse (16 - 40A)
100A Fuse (63 - 100A)
Fault breaking capacity:
3,000A
When used in conjunction with a Sentry MCB, the RCD
will withstand fault current up to the breaking capacity
of the MCB, i.e. 6,000A.
When backed up by a BS 88 fuse, then the breaking
capacity of the RCD is increased to 10,000A.

I'm a little confused by the fact that the breaking capacity of the RCD depends upon the associated OPD - if the RCD operated first, it would surely have to break the same fault current, regardless of what OPD was in the circuit? (and if the OPD operated first, it wouldn't have to break any current at all!)

Kind Regards, John

 

[Adam_151]

I think that it'll be down the fact that the fuse effectivly attentuates the measured value of fault current John.

I.e. If you measured 10kA. the fault would never actually rise to 10kA due to, the impedance of the supply (this will be aided by the coils in the RCD) causing the fault current to rise in a steep gradient, rather than 'instant on' the fuse will likely clear the fault before 10ka is reached, in addition the heat generated in the fuse element will increase the resistance, further lowing the fault current

Dorman smith used to offer a fault current limiter for their 5A loadmaster breakers. It increased the breaking capacity by 1kA. It consisted of a piece of metal which screwed to the output of the MCB with several sharp turns in it

 

[JohnW2]

I think that it'll be down the fact that the fuse effectivly attentuates the measured value of fault current John.....

I'm not sure that I really understand that. The implication of what the spec says seems not to be that the presence of an OPD reduces the fault current but, rather, that the fault-breaking capacity of an RCD increases in the presence of an OPD.

... I.e. If you measured 1000kA. ...

Do you really mean that? - it would imply a fault loop impedance of 0.00023Ω at 230V (aka about 130 cm of a pair of 25mm² conductors)!

Kind Regards, John

 

[scousespark]

Eric posted this and it is wrong.

Is the RCD considered as forming a circuit? From the above I would say yes.

Circuit. An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s).

If you exceed 30 mA to earth is that "overcurrent"? If so then the RCD's form a circuit.

Read more: http://www.diynot.com/diy/threads/limitation-of-63a-rcd.435599/#ixzz3bizFTIDP

An RCD does not detect overcurrent. It detects an imbalance between Line and Neutral. This is referred to as the earth fault current and the RCD is designed to trip at a certain value (typically 30ma in domestics CUs). The overcurrent protection is provided by the MCBs linked to the RCD.

We don't apply diversity, but the reality is that it is unusual for an installation to be fully loaded. A 63A RCD will function with currents in excess of this, up to a point where it burns out.

The process to follow is to calculate the loads and from this design the circuits. You then select the CU based on the requirements. If you just pick the CU and design from there, you are disadvantaged from the outset.