RCD's and RCBO's...Whats the difference you may ask?

[holmslaw]

There has and will always be confusion over rcd's, because the term rcd refers to a device that operates only on earth leakage sensing, but rcd is also a generic term for any device that uses earth leakage sensing.

Rcd's that only operated by earth leakage sensing used to be called rccb's (residual current circuit breakers) but for some reason the name was dropped. They were originall called elcb's an acronym that does'nt need explaining.

Then rcbo's (residual corrent breaker with overload) came along combining an rcd and mcb in one unit, and it was decided that they were a type of rcd.

So an rcd or rccb only operates on earth leakage, and an rcbo is also called an rcd.

 

[ban-all-sheds]

An RCBO is an RCD and provides integral overcurrent protection, ergo certain types of RCD do provide overcurrent protection like I said. This does not imply that it is the RCD function of the RCBO providing it, of course.

No - an RCBO is a device which provides, inter alia, RCD functionality.

RCD functionality does not include overcurrent protection.

RCDs do not provide overcurrent protection.


Edited to clarify that this is a reply to Risteard.

 

[ericmark]

I read on the other Electrical forum an old discussion about RCD's and American GFI's. In this discussion it was stated that RCD's do not give Overload protection, however I would like to point out that this is wrong, RCD's DO give overload protection, only basic, but it is there.

I hope you never do work for anyone I know. The leakage current is not the load so it does not give any overload protection.
RCD's and Isolators can look very like MCB's and I have seen both used where the guy thought they were giving overload protection. The Loadmaster range of MCB's had one with red operating bar with 100A marked on it which was purely an Isolator but looked very similar to the MCB's

As to DC sensing the "A" type will sense DC but it does need to move through a zero volts in wave form to work. The "AC" type will not work with DC. There are specials that will work with pure DC but these are very special.

There are many types of RCD / RCBO from active to passive to multi-pole switching and single-pole switching. They will be marked with a number of ratings. The let-through amps they can handle often around the 4700 amp. The working amps they can handle for example 63A and the tripping amps for example 0.03A and the time to trip for example under 40ms or S type. Some will allow these setting to be altered and they can be in three sections the control unit with time and amp pots for setting the current transformer and the mounded breaker used to open the circuit in event of a fault.

When combined as with last example with a moulded or miniature breaker then they will also have another two devices a thermal overload and a magnetic overload.

However passive RCD's do not fail safe. It does not matter if fitted to moulded breaker or a single pole switching RCBO as used in domestic if the voltage falls below a threshold or a wire becomes disconnected they may fail to work. The test button only tests the mechanics of the device and it needs a special meter to test the time they take to trip.

The domestic single pole RCBO has a draw back in that it does not open all live conductors only the line conductor is opened as a result their use is restricted to a selected earthing arrangement and can't be used in all premises.

Some RCBO are sold in two parts and there must be an issue there where they can be assembled wrong and even when supplied as one using heavy cables and incorrect buss bars can warp the case preventing them from working so all RCD devices should be tested after fitting and I don't mean the test button.

 

[ericmark]

A motor car is a motor vehicle but a motor vehicle is not necessary a motor car it could be a wagon.

So in same way a RCBO is a RCD but a RCD may not be a RCBO.

Years ago I worked with mounded breakers with the trip solenoid fitted and a control box and current transformer we never called these RCBO's it was a overload device with a RCD added.

I agree with "holmslaw" the old wording was better.

 

[ban-all-sheds]

I hope you never do work for anyone I know.

1) You should look at some of his other posts, if you have a few hours to kill - by his standards this one was fairly competent. If searching, he was also known at some time as FWL_Engineer, and used the name "Jim Franklin", but not AFAIR, as an actual user ID.

2) Don't worry anyway, he's dead now, hurrah.

(Guess I should clarify - I'm not actually rejoicing in his death - he lied about that on another forum - either pretending to be, or getting someone else to pretend to be, his brother reporting his sad death in a road accident, but at least him pretending to be dead means we're now spared his desperate ramblings and rantings)

 

[Risteard]

Rcd's that only operated by earth leakage sensing used to be called rccb's (residual current circuit breakers) but for some reason the name was dropped.

They are still correctly called RCCBs.

RCCBs, RCBOs, SRCDs and others are all equally RCDs.

Therefore anyone disagreeing with me that an RCBO is an RCD is frankly wrong.

And as such, whilst it is not a function of an RCD to provide overcurrent protection it cannot be said that no RCD provides overcurrent protection.

RCD (Residual Current Device) is a GENERIC term for such devices. It does not relate to a specific type of device such as an RCCB.

 

[Risteard]

So in same way a RCBO is a RCD but a RCD may not be a RCBO.

Correct. It's really quite straightforward.

 

[ricicle]

We all know it's straightforward, but by playing with the semantics we can end up confusing those who are trying to understand.

BS7671:2008 - Part 2 Definitions. There are two descriptions - one for RCD and one for RCBO .

 

[Risteard]

BS7671:2008 - Part 2 Definitions. There are two descriptions - one for RCD and one for RCBO .

Why shouldn't there be?

One is the definition of the generic term, and the other is the definition of a specific type of RCD - namely an RCBO.

 

[ban-all-sheds]

And the definition for RCD does not include anything about overcurrent protection.

 

[Risteard]

And the definition for RCD does not include anything about overcurrent protection.

No-one said it did. Doesn't change the fact that an RCD can have integral overcurrent protection when it is an RCBO, which has integral overcurrent protection.

That's not open to debate.

 

[ban-all-sheds]

Look up Definitions (Part 2).

I have.

An RCD does not provide overcurrent protection.

 

[Risteard]

An RCD does not provide overcurrent protection.

No-one said it did. Doesn't change the fact that an RCD can have integral overcurrent protection when it is an RCBO, which has integral overcurrent protection.

That's not open to debate.

 

[Grizzly]

I'm afraid Risteard is correct.

The definition given for RCD covers any device (or association of devices) that provides residual current protection.

The definitions of RCBO and RCCB specify if overload and/or short circuit protection is included or not.
Both an RCBO and an RCCB would be variations of an RCD.

 

[LoveRocket]

But that does not mean that an RCD provides overcurrent protection.

Look up Definitions (Part 2).

Erm, where did Risteard say it did?

Of course an RCBO is an RCD, just as an RCCB is an RCD.

Anyone who claims that a Residual current Circuit Breaker with integral Overcurrent protection is not a Residual Current Device is clearly an idiot.