Hi anyone know where I can find the max zs of a 2amp C type rcbo?
Max zs of 2amp C type rcbo?
- Thread starter traineelectrician
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230 / (2 x 10)
Do you really mean 2 amp? [If so, I agree with the answers you've been given]
Kind Regards, John
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Ring the manufacturer & ask them!!!
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Looking at Zs to allow magnetic part to trip I make it 11.5 ohms using 230 volt or 12 ohms using 240 volt. B = x5, C = x10 and D = x20 so a C2 will need 20 amps to trip the magnetic part so simple ohms law 230/20 = 11.5 ohms.
However since RCBO then TABLE 41.5 NOTE 2:* The resistance of the installation earth electrode should be as low as practicable. A value exceeding 200 ohms may not be stable. Refer to Regulation 542.2.2.
So the Zs should be 200 ohms. As to R1 - Rn I see nothing requiring a fault to cause the supply to open within a short time and likely one could rely on the thermal part of the trip. But then one must ask why include a magnetic part with a RCBO if not required? So I would assume I have just not found the reference and the line - neutral loop impedance should be below 11.5 ohms?
Where a single pole RCBO is used with a S-type feed to RCBO with TT system then could be looking at a Zs of 150 ohms approx.
However since RCBO then TABLE 41.5 NOTE 2:* The resistance of the installation earth electrode should be as low as practicable. A value exceeding 200 ohms may not be stable. Refer to Regulation 542.2.2.
So the Zs should be 200 ohms. As to R1 - Rn I see nothing requiring a fault to cause the supply to open within a short time and likely one could rely on the thermal part of the trip. But then one must ask why include a magnetic part with a RCBO if not required? So I would assume I have just not found the reference and the line - neutral loop impedance should be below 11.5 ohms?
Where a single pole RCBO is used with a S-type feed to RCBO with TT system then could be looking at a Zs of 150 ohms approx.
Indeed.
Only if, as you imply, it is a TT installation. Otherwise, one is not allowed to rely upon 'an RCD' (and I presume that includes the 'RCD part' of an RCBO) for protection against L-E faults - with TN supplies, Zs would therefore have to be no greater than 11.5 Ω for the C2 (as above).
I don't quite understand that statement.
Kind Regards, John
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I would agree that the regulations are not clear as to the use of an RCD. 411.4.4 permits the use of an RCD for fault protection but it also must have an over current device. But it does not say which device has to comply with disconnection times and the fact that 106 meters is quoted as limit of cable in a ring main worked out by volt drop rather than earth loop impedance since 2008 one would assume the magnetic part of the RCBO it not required to be operative.
However where line - neutral faults occur where the loop impedance is low one would want fast disconnection times. So one would still include the magnetic part for when the impedance is low.
I a long time ago raised the use of C32 RCBO devices in final rings where an impedance of 0.72 ohms would be required to operate the magnetic part of the device. With the reliable impedance of the incoming TN-C-S supply being taken at 0.35 ohms that leaves just 0.37 for the installation final rings would need to be far shorter than the 106 meters to comply with that.
I know the RCD is an additional device, but we are not saying the current overload should not be there, just asking which device is required to disconnect within the stated times with an earth fault.
With a TT system we really need to disconnect all live conductors and one questions the use of single pole RCBO's even where a 100ma S type is used as main isolator to fit single pole RCBO's is questionable.
411.3.3 Note 2 shows we don't need RCD protection with FELV or reduced low voltage but reading 522.6.5 to 522.6.7 it would be easy to miss that and this is the problem with BS7671:2008 one ends up flitting back and to many times to pick up all the points made.
This is why I did not state it as a fact but questioned if we should rely on the disconnection times of the RCD. We are not relying on the RCD as only disconnection device but we are relying on the RCD to comply with disconnection times.
I am unclear as to the validity of my points and would welcome any other comments I hope with regulation references.
As to 150 ohms comment where for example a 300mA S type RCD is used the ELI drops below the 200 ohms but on reflection being an S type it would not comply with disconnection times anyway so really it's more down to if single pole RCBO's should be used on a TT supply.
But back to nitty gritty does one take the 11.5 ohms or the 200 ohms as the figure to use for the ELI with a C2 RCBO?
A with TN
B with TT
Now time to stand back and read replies.
However where line - neutral faults occur where the loop impedance is low one would want fast disconnection times. So one would still include the magnetic part for when the impedance is low.
I a long time ago raised the use of C32 RCBO devices in final rings where an impedance of 0.72 ohms would be required to operate the magnetic part of the device. With the reliable impedance of the incoming TN-C-S supply being taken at 0.35 ohms that leaves just 0.37 for the installation final rings would need to be far shorter than the 106 meters to comply with that.
I know the RCD is an additional device, but we are not saying the current overload should not be there, just asking which device is required to disconnect within the stated times with an earth fault.
With a TT system we really need to disconnect all live conductors and one questions the use of single pole RCBO's even where a 100ma S type is used as main isolator to fit single pole RCBO's is questionable.
411.3.3 Note 2 shows we don't need RCD protection with FELV or reduced low voltage but reading 522.6.5 to 522.6.7 it would be easy to miss that and this is the problem with BS7671:2008 one ends up flitting back and to many times to pick up all the points made.
This is why I did not state it as a fact but questioned if we should rely on the disconnection times of the RCD. We are not relying on the RCD as only disconnection device but we are relying on the RCD to comply with disconnection times.
I am unclear as to the validity of my points and would welcome any other comments I hope with regulation references.
As to 150 ohms comment where for example a 300mA S type RCD is used the ELI drops below the 200 ohms but on reflection being an S type it would not comply with disconnection times anyway so really it's more down to if single pole RCBO's should be used on a TT supply.
But back to nitty gritty does one take the 11.5 ohms or the 200 ohms as the figure to use for the ELI with a C2 RCBO?
A with TN
B with TT
Now time to stand back and read replies.
I think we discussed this one recently, didn't we lads?
The answer was that BS 7671 permits the use of an RCD on circuits within TN installations where the Zs cannot be achieved, I believe. It must be the designer to decide what that means and what are the risks associated with relying on an RCD to trip within the required time period are, if any.
Then one must look at the chapter relating to overcurrent protection. Last time, I did a few graphs for different scenarios of cable configuration, and worked out the maximum additional cable length (and therefore R1+Rn) you could add above the permitted Zs level for a standard MCB curve, to stay within the limits of the adiabatic equation. The two curves where plotted together
The results were that once out of the magnetic part of the curve, a couple of popular standard circuits give up relatively quickly.
Unfortunately I did not save the results, but my conclusion was that I suspect the failure of a popular circuit to meet the adiabatic with only a small increase in cable length over the max MCB curve Ze was the reason for the guidance to stick with the firm rule on TN circuits that you must stay within the limits of the magnetic/thermal MCB impedance limits. An additional, historic reason may be that MCBs have traditionally been protected by RCDs in groups, rather than individually, and although this was permitted and unavoidable for TT circuits, the addittional risks of multiple circuits going off is not really acceptable for a TN circuit where such a failure is widely avoidable.
The answer was that BS 7671 permits the use of an RCD on circuits within TN installations where the Zs cannot be achieved, I believe. It must be the designer to decide what that means and what are the risks associated with relying on an RCD to trip within the required time period are, if any.
Then one must look at the chapter relating to overcurrent protection. Last time, I did a few graphs for different scenarios of cable configuration, and worked out the maximum additional cable length (and therefore R1+Rn) you could add above the permitted Zs level for a standard MCB curve, to stay within the limits of the adiabatic equation. The two curves where plotted together
The results were that once out of the magnetic part of the curve, a couple of popular standard circuits give up relatively quickly.
Unfortunately I did not save the results, but my conclusion was that I suspect the failure of a popular circuit to meet the adiabatic with only a small increase in cable length over the max MCB curve Ze was the reason for the guidance to stick with the firm rule on TN circuits that you must stay within the limits of the magnetic/thermal MCB impedance limits. An additional, historic reason may be that MCBs have traditionally been protected by RCDs in groups, rather than individually, and although this was permitted and unavoidable for TT circuits, the addittional risks of multiple circuits going off is not really acceptable for a TN circuit where such a failure is widely avoidable.
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I think the answer is dependent on why the question is asked. Remember he calls himself "traineelectrician" so this could be part of a college question and if it is then he will require the references to the IET/BSI regulations.
I personally would consider the 200 ohms limit was good enough but as to quoting regulations to support this that's another question.
So back to "traineelectrician" is this a college question or something you need to do for work.
I personally would consider the 200 ohms limit was good enough but as to quoting regulations to support this that's another question.
So back to "traineelectrician" is this a college question or something you need to do for work.
I would have thought that. particularly if it were a college question, the answer expected would probably be calculated as U0/Ia, wouldn't it?
You keep saying that, but as an unqualified answer to the question as posed, it would surely not be appropriate, and particularly risky if one were answering a college question. Apart from anything else, it's really only appropriate in terms of a TT installation (and there's no mention of that in the question posed), not the least because a Zs of 200Ω would be a cause for considerable concern in any TN installation.
Kind Regards, John
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