Maximum ZS exceeded on RCBO

[newfutile]

On a recent EICR i have ring final circuits (sockets) on type C 32 amp rcbo
Some of my ZS reading exceed maximum ZS (0.55 OSG) ie:0.65 ohms .

On a earth fault this will disconnect via the MCB part of the RCBO.
How can i ensure a LINE to Neutral fault will still disconnect?

 

[i=p/u]

Change breaker or increase cable simples.

 

[Strima]

Was the Zs measured or calculated?

Was it tested using a no trip setting?

Any dead test results (R1+R2)?

What's the Ze?

PFC?

Did you do a Zn measurement?

 

[westward10]

All you need worry is ADS complied with, yes.

 

[Rockingit]

What protective purpose is the RCBO serving?

 

[telectrix]

What protective purpose is the RCBO serving?

steak and chips, followed by raspberry cheesecake.

 

[loz2754]

It would be interesting to know why type C RCBOs have been used. However, as @westward10 says, ADS has been complied with in this case, as the max Zs is 1667 ohms for a 30mA RCD.

 

[newfutile]

It’s for additional protection , and yes the ZS have been measured directly and calculated ZE+R1+R2 PFC 3.08KA on 3phase at DB.

 

[Strima]

Was it tested using the no trip setting on the meter?

 

[newfutile]

Yes it was , but also as dead test + ZE

 

[HappyHippyDad]

As one or 2 others have inferred, you have probably adhered to the regulations and fault protection simply by using an RCBO. If you use the RCBO for fault protection then you only need a Zs value <1667ohms. This is acceptable as per reg 411.4.5. However, I can't quite get my head around being able to just use the RCD and ignore the max Zs figures on a TNS or TNCS.

 

[newfutile]

I think its something that’s overlooked during testing , in fact the neutral itself gets very little testing done on it

 

[pc1966]

On a recent EICR i have ring final circuits (sockets) on type C 32 amp rcbo
Some of my ZS reading exceed maximum ZS (0.55 OSG) ie:0.65 ohms .

As other have pointed out, the RCD aspect will disconnect (if it is working) for a wide range of Zs value.

Normally I do not advise relying on a single RCD for disconnection, however, here you are only a little above the OSG Zs limits and when you consider they take the upper MCB trip point, the lowest (almost) mains voltage, and allow 20% for heated conductors, you see there is a lot of margin built in.

So in your place with just the single RCD function of the RCBO guaranteeing ADS I would not worry, as the OCPD aspect will probably meet it (and certainly would if you have a couple of seconds for the thermal trip to go).

While the typical 30mA RCBO is theoretically compliant to 1.67k Zs (from 50V/30mA) the only excuse for anything other than a couple of ohm Zs is Ze being high due to a TT earth rod, and then it should not exceed 200 ohms (from regs 411.5.3 'NOTE 2' below Table 41.5)

On a earth fault this will disconnect via the MCB part of the RCBO.
How can i ensure a LINE to Neutral fault will still disconnect?

Usually if you do a Zs measurement at the end of a circuit you normally get both the PFC and the PSSC from most MFT. If the PSSC is high enough then your L-N loop is fine for disconnection. In the general C-curve MCB that is 10 times the rated In value, though you should make some allowances for low volts and heated conductors.

If your PSSC is 20 times the rated current (load current for fixed fixed loads, the MCB 'In' rating for general/socket systems) then you have also verified the 5% volt drop requirements.

As a general rule, if the PSSC is significantly less than the PFC you should check. If that is due to metal trunking, conduit, and other parallel earth paths, then fair enough. But if no explanation is found then it is a reason to check the Rn as well as R1 values independently.

 

[pc1966]

If your PSSC is 20 times the rated current (load current for fixed fixed loads, the MCB 'In' rating for general/socket systems) then you have also verified the 5% volt drop requirements.

Just to add, the "20 times" test is verifying the total supply VD, not just the final circuit, so it is more demanding than often used for design.

For final circuits that are a small fraction of the DB supply rating it is more or less the same. But for circuits that are a significant portion of the DB then you might not need to meet 20x as somewhere there would be an allowance for the supply VD, and separately for the circuit VD.