type C rcbo's

[steptoe]

^^^^

:slap

 

[NozSpark]

Wowzers:slap

 

[Nicky Tesla]

:C

 

[Steve3948]

I think the confusion is that the OP did not state if the installation was TT or TN, with 411.5 dealing with TT and table 41.5, and 411.4 dealing with TN and table 41.3 to be read with 411.4.9 where it states "Where an RCD is used to satisfy the requirements of Regulation 411.3.2.2 the maximum values of earth fault loop impedance in Table 41.5 may be applied for non-delayed RCD's to BS EN 61008-1 and BS EN 61009-1 for a nominal voltage of 230V. In such cases, an overcurrent protective device shall provide protection against overload current and fault current in accordance with chapter 43"

 

[park1]

I think the confusion is that the OP did not state if the installation was TT or TN, with 411.5 dealing with TT and table 41.5, and 411.4 dealing with TN and table 41.3 to be read with 411.4.9 where it states "Where an RCD is used to satisfy the requirements of Regulation 411.3.2.2 the maximum values of earth fault loop impedance in Table 41.5 may be applied for non-delayed RCD's to BS EN 61008-1 and BS EN 61009-1 for a nominal voltage of 230V. In such cases, an overcurrent protective device shall provide protection against overload current and fault current in accordance with chapter 43"

And that's where the Zs value comes into play.

 

[sparkytim]

How does the Zs value have a bearing on a L-N short circuit fault?

 

[park1]

You need to look at the Curve characteristic for a MCB. If a short between L-N occurs the device has to operate in a time which does not exceed the Temp limit of the given cable. With An MCB it move very quickly from Instantaneous to many second's, hence the thermal constraints may not be met. Meet the Zs value and you know you will have instantaneous operation.

 

[Steve3948]

So the Line Impedance for a C Type RCBO in order to trip in 0.4sec is 0.7 ohms, a TNS system should be no more than 0.8 ohms.

That's how I see it, anything less than 0.7 ohms will give you the disconnection within 0.4 sec.

 

[sparkytim]

I think you need to check up on the definition of Zs

 

[Steve3948]

I think you need to check up on the definition of Zs

Who me ????

 

[sparkytim]

I think we may have some confusion between Ze and Zs and also with others what type of fault is affected by their values.

For TNC-S systems the Ze will be effectively equal to the L-N loop value, however for TNS and TT they will not.

 

[park1]

I think you need to check up on the definition of Zs

No Tim, i don't lol.

Ok, What's the Zs value of a MCB achieve disconnection time wise?

Shall we Use I2(Current causing effective operation of device)?

Think about the Adiabatic

 

[Steve3948]

I did a test and inspection on a domestic premises and noticed the RCBO's installed were type C, these have a much lower Zs value than the type B, apart from that i always assumed type C's were for circuits with equipement that had a high start up current. Are type C RCBO's acceptable in domestic installations?.

So lets get back to the first post by LNRDF01, is it ok or not !!!!!

I say YES in most cases.

 

[sparkytim]

And I would agree

The Onsite guide page 56 Table 7.1(i) lists a 32 Amp RCBO Type C Ring final circuit protective device.

For TN-C-S system it gives a max circuit length of 133 mtrs and then you note sc adjacent.

A look on page 65 at the notes shows sc to mean "Limited by LINE to NEUTRAL loop impedance (short circuit)

 

[park1]

And I would agreeThe Onsite guide page 56 Table 7.1(i) lists a 32 Amp RCBO Type C Ring final circuit protective device.

For TN-C-S system it gives a max circuit length of 133 mtrs and then you note sc adjacent.

A look on page 65 at the notes shows sc to mean "Limited by LINE to NEUTRAL loop impedance (short circuit)

If you look at the curves Tim, move from the instantaneous to the left, If the Live loop is high teh the disconnection quickly becomes seconds, put that into the Adiabatic and see if the Thermal Constraints of the cable are satisfied.

 

[sparkytim]

Yes thats true the L to N Loop not Zs

 

[LNRDF01]

thanks steve, we eventually got to the main question

 

[park1]

Yes thats true the L to N Loop not Zs

Im not referencing the L-N loop as Zs, im saying you can use the Zs values for a MCB in regard to Thermal Constraint's, i think its a lack of understanding by others, otherwise they would of understood what was being said.

 

[Steve3948]

Im not referencing the L-N loop as Zs, im saying you can use the Zs values for a MCB in regard to Thermal Constraint's, i think its a lack of understanding by others, otherwise they would of understood what was being said.

$park I think we are fully aware what you putting over with regard to using the Zs value for a MCB in regard to Thermal Constraint's, however the first post asked if a 32A C Type RCBO feeding a Ring Main wired in 2.5mm

 

[park1]

Not sure I follow Steve, I draw out Adiabatic lines for various cables, this allows me to see if the thermal constraints have been met for all conductors.