Max permissible ZS for rcbo 61009

[atm84]

So, when I did my 2391 I&t course I was told to use to values in table 41.3 of the regs for max ZS value for rcbo which provide earth fault, and over load protection. Recently a colleague of mine said you should use the value in table 41.5 which states a max permissible ZS of 1667 ohms. I don't think that is correct as I didn't think table 41.5 is correct where the device is also providing overload protection.

Any thoughts?

 

[Plonker 3]

What is the earthing system?

On a new install with a TN system then the 1667 should not apply as the RCBO is used for additional protection

 

[atm84]

It's a TNC-S system. I agree. I thought table 41.5 was in relation to TT systems as it comes under that title, and that it was relating to rcd devices that are only providing earth fault protection and not over load protection as well.

My mate put 1667 ohms on the test cert for an rcbo. We ended spending half the day discussing it. Grrrr

 

[Plonker 3]

If it was a EICR I would say if the Zs is above the max allowed, code it accordingly and put it requires further investigation.

 

[atm84]

It's a new install and the measures ZS is well within the max allowed for whichever value you use. It was just an observation on what value you put in the column for max permissible value

 

[telectrix]

if it's a TN system, then the measured Zs's should not exceed the values given for the OCPD part of the RCBO. and these are the values you should use. the 1667 value should only be used where RCDs are used for fault protection. i.e. on a TT system. ( that's my view and i'm sticcking to it, no matter what nic-nic, scalp-it, or erecta say).

 

[atm84]

Thanks Telectrix

 

[yellowvanman]

411.4.9

 

[Deleted member 9648]

411.4.9

What he said.
Like it or not fella's 411.4.9 covers TN systems and states that 'where an RCD is used to satisfy 411.3.2.2 (disconnection times) table 41.5 may be applied.
Therefore either table may be used as the maximum Zs for an RCBO.

 

[D Skelton]

There is no dispute here. The regs are very clear on the maximum permissable Zs values for any type of OCPD or RCD it covers on any system within the remit of BS 7671.

I assume we are talking here about a 30mA RCBO? No one has made this clear.

If so, the maximum permissable Zs value on a TN system is 7667ohms. On a TT system the maximum permissable Zs value is 1667ohms. The word 'maximum' is the clue here!

Regardless of how ridiculous these values are, regardless of whether or not you would accept such measured values and regardless of what you think may or may not be best practice, these are the cold hard facts.

 

[atm84]

Ok but if the device is providing overload protection then surely you would use the values in table 41.3. Table 41.5 does not make reference to a device that is providing overload protection.

 

[atm84]

What he said.
Like it or not fella's 411.4.9 covers TN systems and states that 'where an RCD is used to satisfy 411.3.2.2 (disconnection times) table 41.5 may be applied.
Therefore either table may be used as the maximum Zs for an RCBO.

But that is relating to RCDs that are providing earth fault protection. I am referring to an RCBOs that is providing earth fault and over current protection.

 

[yellowvanman]

Ok but if the device is providing overload protection then surely you would use the values in table 41.3. Table 41.5 does not make reference to a device that is providing overload protection.

411.4.9 covers that and Chapter 41 is generally dealing with protection against electric shock not overload, that is covered in Chapter 43!

 

[D Skelton]

Ok but if the device is providing overload protection then surely you would use the values in table 41.3. Table 41.5 does not make reference to a device that is providing overload protection.

What has overload protection got to do with earth fault protection? The maximum permissable Zs value is there to ensure that disconnection times are met in the event of an earth fault only. There is no maximum permissable disconnection time for short circuit or overload, therefore that part becomes irrelivant.

The maximum permitted Zs of a 32A B type 60898 to ensure that a disconnection time of less than 0.4s is achieved on a TN system in the event of an earth fault is 1.15ohms.

The maximum permitted Zs of a 30mA 32A B type 61009 to ensure that a disconnection time of less than 0.4s is achieved on a TN system in the event of an earth fault is 7667ohms.

 

[atm84]

What has overload protection got to do with earth fault protection? The maximum permissable Zs value is there to ensure that disconnection times are met in the event of an earth fault only. There is no maximum permissable disconnection time for short circuit or overload, therefore that part becomes irrelivant.

The maximum permitted Zs of a 32A B type 60898 to ensure that a disconnection time of less than 0.4s is achieved on a TN system in the event of an earth fault is 1.15ohms.

The maximum permitted Zs of a 30mA 32A B type 61009 to ensure that a disconnection time of less than 0.4s is achieved on a TN system in the event of an earth fault is 7667ohms.

Ok I can hear your argument but that is contrary to what I was told on the 2391 course and what most other electricians I have spoken to say. However, my colleague agrees with you but states max ZS should be 1667 ohms and not 7667 ohms that you state above.