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

[sxdave]

I would appreciate some opinions on the following situation...
I have been asked to comment on an EICR for a local community resource (church hall used by various groups of all ages).
During an EICR on the TN-C-S installation the testing of a ring final circuit gave R1+R2 and Zs(db) values that when added together exceeded the maximum acceptable for the MCB type and current rating. This would imply the RCD was required for fault protection.

When Zs measurements were made around the ring the maximum measured Zs value was found to be lower than the required value for satisfactory fault protection by the MCB. This would imply the RCD was present only for additional protection.

I assume the Zs readings from around the ring are lower because of a fortuitous parallel earth path through a gas and/or water pipe. I consider this an earth path, however it comes about, to be one that cannot be relied upon as it is not part of the electrical installation and could change (eg a metal pipe could be replaced by plastic) so my inclination is that the RCD as necessary for fault protection. The EICR records it as for additional protection only.

Opinions on this?

 

[UNG]

Is it only one circuit where the R1 + R2 and Zs values added together give a value that exceeds the maximum Zs allowed, if so this would suggest that this circuit needs further investigation as there could be a loose or poorly made connection to one or more of the sockets or even the switches on the sockets causing a higher than normal R1 + R2 value to be measured

If you post up the EICR with identifying details redacted I'm sure you will get many comments from the members

 

[Electro-tech]

I would appreciate some opinions on the following situation...
I have been asked to comment on an EICR for a local community resource (church hall used by various groups of all ages).
During an EICR on the TN-C-S installation the testing of a ring final circuit gave R1+R2 and Zs(db) values that when added together exceeded the maximum acceptable for the MCB type and current rating. This would imply the RCD was required for fault protection.

When Zs measurements were made around the ring the maximum measured Zs value was found to be lower than the required value for satisfactory fault protection by the MCB. This would imply the RCD was present only for additional protection.

I assume the Zs readings from around the ring are lower because of a fortuitous parallel earth path through a gas and/or water pipe. I consider this an earth path, however it comes about, to be one that cannot be relied upon as it is not part of the electrical installation and could change (eg a metal pipe could be replaced by plastic) so my inclination is that the RCD as necessary for fault protection. The EICR records it as for additional protection only.

Opinions on this?

Why are you adding R1 + R2 and Zs(db) together or is the Zs(db) a typo meaning Ze( db). No offense Zs = Ze + R1 + R2.
Is the DB remote from the main intake?

 

[sxdave]

Why are you adding R1 + R2 and Zs(db) together or is the Zs(db) a typo meaning Ze( db). No offense Zs = Ze + R1 + R2.
Is the DB remote from the main intake?

Thanks for taking look at this. Yes, the DB is quite a distance from the main intake, about 5-6 metres horizontally. So given ceiling height the run of cable is probably more like 8-10 metres

 

[sxdave]

Why are you adding R1 + R2 and Zs(db) together or is the Zs(db) a typo meaning Ze( db). No offense Zs = Ze + R1 + R2.
Is the DB remote from the main intake?

Ze measured at the intake as 0.15 ohms, R1+R2 of cable from intake to db measured as 0.04 ohms, Zs(db) is recorded on EICR as 0.19 ohms. Final ring circuit R1+R2 measured value is 0.45 ohms. MCB is 32 amp 'C' curve.
So I can do the calculation Zs = Ze + (R1+R2 [intake to DB]) + (R1+R2 [ring final act])

 

[sxdave]

Is it only one circuit where the R1 + R2 and Zs values added together give a value that exceeds the maximum Zs allowed, if so this would suggest that this circuit needs further investigation as there could be a loose or poorly made connection to one or more of the sockets or even the switches on the sockets causing a higher than normal R1 + R2 value to be measured

If you post up the EICR with identifying details redacted I'm sure you will get many comments from the members

Thanks for replying. Here are some of the relevant measurements...
Ze = 0.15 ohms
R1+R2 of cable from intake to DB is 0.04 ohms (DB is about 5-6 metres horizontally distant from the intake).
R1+R2 of ring final circuit is 0.45 ohms.
MCB is 32 amp, 'C' curve.
Max Zs measured at sockets on ring is 0.38 ohms.

So calculations I do are
1) Max acceptable impedance for the MCB to meet disconnection time requirement is 0.8 x 0.68 = 0.544 ohms.
2) Zs from measured Z and R1+r2 values is 0.15 + 0.04 + 0.45 = 0.64 ohms.

 

[sxdave]

Is it only one circuit where the R1 + R2 and Zs values added together give a value that exceeds the maximum Zs allowed, if so this would suggest that this circuit needs further investigation as there could be a loose or poorly made connection to one or more of the sockets or even the switches on the sockets causing a higher than normal R1 + R2 value to be measured

If you post up the EICR with identifying details redacted I'm sure you will get many comments from the members

Sorry, missed one point in your reply. There is one other circuit where the values of Zs(db) and R1+R2 lead me to question the true purpose of the RCD. I do not think there is a loose connection given the values recorded.

 

[Julie.]

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.

 

[UNG]

Sorry, missed one point in your reply. There is one other circuit where the values of Zs(db) and R1+R2 lead me to question the true purpose of the RCD. I do not think there is a loose connection given the values recorded.

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

 

[Julie.]

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

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!

 

[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!