RCBOs vs RCDs Pt2

[Engineer54]

You’ve changed your tune.

The trouble is the SP RCBO just don’t match what it says on the tin. For this reason I think many have accepted them as an instant cure all in every situation. This can only be attributed to lack of knowledge on the part of the electrician. (The ones that bite my head off now will be the ones in the know.)

Some manufacturers do point out the short comings of the SP RCBO in handling a N→E fault. To my mind it should be made clear to everyone. As we’ve often discussed it seems to be the doctrine of colleges to preach fitting RCD’s to everything.

How have i changed my tune??

 

[Deleted member 26818]

Current flowing through the cpc from the cutout in normal operation ???
Where did you get that one from?

The only thing flowing in a cpc under normal conditions will be leakage current from smps's and similar.

I give up.

 

[Geoffsd]

It's similar to this-

 

[davesparks]

I give up.

Have a look at the picture Geoff posted, it's not that clear but does show the diverted current

 

[Deleted member 26818]

Have a look at the picture Geoff posted, it's not that clear but does show the diverted current

I know how it works, just having difficulty making you understand.

 

[davesparks]

I understand perfectly well thankyou, I'm not sure you do.

Neutral current from the whole installation diverts through an N-E fault on a circuit.

Current does not normally flow through a cpc

 

[Deleted member 26818]

I understand perfectly well thankyou, I'm not sure you do.

Neutral current from the whole installation diverts through an N-E fault on a circuit.

Current does not normally flow through a cpc

Like I said, I give up.

 

[Engineer54]

Makes you wonder how we ever managed to get by before the introduction of RCD's, or even before MCB's/MCCB's when fuses were basically the sole means of protection from Main switchboards all the way down to final circuits....

OK, maybe on main switchboards, they could well of had early earth fault relay protection, but then we wouldn't be talking in terms of 30mA or anything close to it!!

Perhaps N-E faults didn't exist in those days!!...lol!!

 

[davesparks]

We got by fine without them, just as we can get by fine without them today if we insisted on better installation standards and stopped treating RCDs as a one size fits all soloution to poor installation work.

Of course neutral faults have always existed, and they will always occur.
But was TNCS, and particularly the PME subdivision of it, so common in those days?

My issue is with the concept of a device which detects a fault on the neutral then reacts by disconnecting the live.
Yes it satisfies bs7671 in terms of protection, but isn't bs7671 supposed to be the minimum standard of acceptability rather than the be all and end all.

 

[Engineer54]

We got by fine without them, just as we can get by fine without them today if we insisted on better installation standards and stopped treating RCDs as a one size fits all soloution to poor installation work.

Of course neutral faults have always existed, and they will always occur.
But was TNCS, and particularly the PME subdivision of it, so common in those days?

My issue is with the concept of a device which detects a fault on the neutral then reacts by disconnecting the live.
Yes it satisfies bs7671 in terms of protection, but isn't bs7671 supposed to be the minimum standard of acceptability rather than the be all and end all.

I'm all for SP+N RCBO's, but as i've stated the present standard SP RCBO's aren't all doom and gloom, they are providing a perfectly acceptable means of additional protection to SP circuits.

Public distribution PME supplies have been around for a lot longer than most imagine, certainly from the middle 1940's with a big expansion in the 50's, 60's, 70's especially in the semi rural areas on the outskirts of towns etc...

 

[davesparks]

SP RCBOs are acceptable yes, but when better options are available for the same price I see no reason to use them in a new board.

Obviously adding or altering a single circuit in an existing board you work with what is there already and if that means fitting SP RCBOs then so be it

 

[Deleted member 9648]

None of which really matters if the RCBO is simply used as an MCB with additional protection.

What on earth are you talking about?

If an mcb is needed you use it as an mcb.

If RCD protection is needed in addition you either install an RCCB before or after the mcb or an RCBO to provide both functions in one.

Whatever reason you install an RCBO for then if it is SP it will detect N-E faults and not disconnect them, it will disconnect the L.

Several cans of worms opened here!!
Personally I think you are over thinking it.
There are countless installations with N-E faults working only on OCPD's not going up in smoke.
An RCBO is normally added to OCPD only installations as a replacement for an MCB but with the benefit of additional protection where bs7671 requires it.

Thats what I'm talking about.