Classification of RCD - for Additional protection only or Fault protection?

[sxdave]

You are querying the purpose of the RCD but not why the RFC is protected by a C curve MCB when swapping to a B curve MCB would solve the problem

I agree, a 'B' curve MCB would eliminate the uncertainty I am seeing. I am trying to clarify how the decision is made in classification of the RCD purpose in a case such as this.
The installation is about 25 years old to the best of my knowledge. Nearly all MCBs are 'C' curve ( 29 out of 34) but no one knows why. Some could be for fluorescent lighting.
Given the age I wonder if the manufacturer of MCBs for the DB is still producing the relevant MCB. I have not checked on this yet.

 

[sxdave]

If ADS cannot be achieved by Zs alone , then yes the RCD is required for fault protection NOT additional protection.

Why Zs is so high could be a myriad of reasons (poor connections etc) , if it is measurably lower than it ought to be (such as measured Zs vs calculated from R1 + R2 etc), then again a myriad of reasons.

If the reason for the measured to be within range is permanent (such as parallel paths due to swa plus a cpc core) then this would be acceptable and RCDs would be additional protection.

If the reason is unknown, or perhaps known but not guaranteed (such as supplementary bonding of pipes) then even if the measured Zs is ok for ADS the RCD would be required for fault protection.

In your case, given the length of the rfc , does the R1 + R2 match the calculated?

Does the Zdb match the Ze plus R1 + R2 of the tails?

It may be as others have stated a poor installation of an otherwise acceptable design, or a poor design in itself.

Thanks for replying.
Your 4th paragraph ('If the reason is unknown....") is of most interest to me regarding this situation. Are you basing this on common sense or is it upon some published material I have missed such as guidance note?

I have not tried to estimate the length of the RFC. I will give that a go.

Yes, Zdb does match Ze + R1+R2 of cable from Intake to DB.

 

[Julie.]

..
Your 4th paragraph ('If the reason is unknown....") is of most interest to me regarding this situation. Are you basing this on common sense or is it upon some published material I have missed such as guidance note?
...

A bit of both (or all three!)

In my initial training it was clear that the system must achieve ADS in the worst possible conditions - low voltage, hot cables, and the minimum amount of earth path which may occur.

And common sense - if I am saying it is safe, it can't be just about ok - as long as a plumber or someone else doesn't do something seemingly unconnected to the electrical system - which leads to an unsafe installation. i.e. replace some copper pipe with plastic or whatever.

The electrical system should be safe on "its own two feet" and not rely on happenstance from other parts.

But, there is something in guidance note 3, it's a bit wishy-washy though and basically goes along the lines of being careful not to include unintended parallel paths with the R2 measurement. So effectively if gn3 says measure R2 without unintended parallel paths, it kind of defines that the confirmation of the R2 portion is using the circuit conductors themselves (or conduit/trunking if they form all or part of the designed R2)

 

[sxdave]

What problem?

It sounds like the installation has ADS achieved by RCD as fault protection, and it is merely the report that mis-defines it as additional protection.

Rather unusual as most mis-identify via ticks as both, or as fault protection when they are actually additional!

The problem is with my understanding of how they decide the RCD is not for ADS. As the RCD is present and functional I consider the circuit to be safe as required by regulations. So, as you surmise, my concern is with the report.

I am very much in favour of your view expressed in one of your posts, copied below in quotes
"The electrical system should be safe on "its own two feet" and not rely on happenstance from other parts."

I raised a number of queries regarding various items that were ticked or not ticked on the EICR. I did this as there seemed to be inconsistenties. I am told by the company that did the EICR that they use a template form with many answers pre-prepared (ticked or not ticked).

 

[Julie.]

The problem is with my understanding of how they decide the RCD is not for ADS. As the RCD is present and functional I consider the circuit to be safe as required by regulations. So, as you surmise, my concern is with the report.

I am very much in favour of your view expressed in one of your posts, copied below in quotes
"The electrical system should be safe on "its own two feet" and not rely on happenstance from other parts."

I raised a number of queries regarding various items that were ticked or not ticked on the EICR. I did this as there seemed to be inconsistenties. I am told by the company that did the EICR that they use a template form with many answers pre-prepared (ticked or not ticked).

That was in relation to UNG's solution of changing to a type B MCB rather than type C to fix the "problem"

I would be concerned about any company that uses "pre-prepared" checklists, as they sound like they have already decided what they will find. Sounds like a drive-by with just enough tweaking to make it look kosher!