Does an RCBo or RCD cover the requirements

[blurt]

When testing a circuit on a consumer unit up-grade and the Zs is higher than the maximum pemissible value does protecting the circuit with a RCD/RCBo meet the requirements of the 0.4 seconds or should the Zs still meet the automatic dissconnection of supply.

 

[Sintra]

Yes if the maximum Zs values for a circuit in a TN system cannot be met, the circuit may be protected by a 30ma RCD. 531.3.1

 

[tornadoarnold]

rcd's and rcbo's work on earth faults, so if you have a short circuit or overload type fault in the circuit it may not disconnect in time. so not really no is your answer.

 

[DurhamSparky]

Yes 30Ma RCD should trip in 0.4 seconds and meet the requirements of a high ZS as you can then use ZS of 1667ohms

 

[tornadoarnold]

guess i need to put my head back in the books

 

[DurhamSparky]

rcd's and rcbo's work on earth faults, so if you have a short circuit or overload type fault in the circuit it may not disconnect in time. so not really no is your answer.

an over load fault rely on the breaker tripping on overload and normally if a fault develops between L-N / L-E it will blow quickly as the fault current will be great!
an RCD earth faullt uses the imbalance of a field and coil to detect leakege to earth!!

 

[JUD]

an over load fault rely on the breaker tripping on overload and normally if a fault develops between L-N / L-E it will blow quickly as the fault current will be great!
an RCD earth faullt uses the imbalance of a field and coil to detect leakege to earth!!

On a circuit with a high Zs a L-E fault current will be lower will it not?

 

[DurhamSparky]

hahahaha...! im off to bed my eyes hurt!!

 

[pevvers]

On a circuit with a high Zs a L-E fault current will be lower will it not?

Indeed it will, hence the need for a maximum Zs, to achieve the required fault current in the event of a fault of neglible impedance. With a high Zs, the chances are you have a high line-neutral loop impedance too.

I do have problems with the idea of "Zs is high, so we'll stick an RCD on it", as it smacks of bad design in the first place...

 

[Deleted member 9648]

As regards overload and short circuit current the requirments are deemed to be met by any of the protective devices permitted by Bs7671....there is a regulation but I cant remember where...


Found ....433.1.2...433.1.3.....435.1

 

[Deleted member 9648]

I do have problems with the idea of "Zs is high, so we'll stick an RCD on it", as it smacks of bad design in the first place...

How do you explain TT systems then?

 

[antimatter]

rcd's and rcbo's work on earth faults, so if you have a short circuit or overload type fault in the circuit it may not disconnect in time. so not really no is your answer.

RCBO - Residual Current Breaker with Overload protection. Combined functions of MCB and RCD in one unit.

 

[pevvers]

How do you explain TT systems then?

TT systems are a worst case scenario, as you cannot get a low Zs so have to resort to using residual current devices for earth fault protection.

But in a TN system, there should really be no need unless the design is rubbish...

 

[Deleted member 9648]

TT systems are a worst case scenario, as you cannot get a low Zs so have to resort to using residual current devices for earth fault protection.

But in a TN system, there should really be no need unless the design is rubbish...

I wouldnt disagree....if a new install it would not be acceptable if the measured Zs exceeded the max value of the protective device chosen at the design stage. But most of these slightly high Zs issues crop up on existing/old circuits being altered or extended,which are none the less perfectly safe ,and IMO using an RCD to meet the disconnection time is perfectly acceptable.

 

[JUD]

On a circuit with a high Zs a L-E fault current will be lower will it not?

Indeed it will, hence the need for a maximum Zs, to achieve the required fault current in the event of a fault of neglible impedance.

Wasn't really a question, more of an observation that DurhamSparky said a L-E fault would blow the OCPD quickly as the fault current would be great. This isn't completely true on a circuit with a high Zs as the fault current would be lower, hence the reliance on an RCD.

 

[Des 56]

Wasn't really a question, more of an observation that DurhamSparky said a L-E fault would blow the OCPD quickly as the fault current would be great. This isn't completely true on a circuit with a high Zs as the fault current would be lower, hence the reliance on an RCD.

I dont fully agree with the wording there JUD
Surely the earth fault curent would be low,but the Short circuit current is usually no different to a Tn system for bumping out the ocpd

The overcurrent on a Tn is more important Zs wise than it is on a TT
The RCD and RCBO s electronics dont like high fault currents apparantly and are not as reliable as on a TT system
The Tn system with an RCD is not as reliable for the function of the RCd as a TT system
OR so I was told

 

[JUD]

I dont fully agree with the wording there JUD
Surely the earth fault curent would be low,but the Short circuit current is usually no different to a Tn system for bumping out the ocpd

If you read my post again, you'll notice that I said a L-E fault (line to earth) on a circuit with a high Zs (Earth fault loop impedance) would not blow the OCPD in the required time but an RCD would.

No mention of short circuit (L-N) which should still have a high enough fault current to blow the OCPD.

 

[pevvers]

No mention of short circuit (L-N) which should still have a high enough fault current to blow the OCPD.

I would agree if the Zs was just over the limits, but not so much if it was ridiculously high, as long as the circuit is T&E.

As the neutral is larger than the CPC, obviously the impedance would be lower on the neutral-line loop. But if it was a ridiculously high Zs, i.e. a couple of Ohms over, then I would be measuring the line-neutral loop to make sure that it is low enough to produce enough fault current to trip the device in the required time....