Maximum permitted Zs

[nickblake]

One of the remidial actions for an installation with a high Zs is to install an RCD so theoretically it doesnt matter what your Zs is as long as its protected by and RCD , having said that even if it is RCD protected i work to the permitted levels .

 

[Ponty Massive]

using the figure of 1667, does this also guarantee disconnection within 0.2s on a TT system?

 

[pushrod]

using the figure of 1667, does this also guarantee disconnection within 0.2s on a TT system?

With the worst scenario fig of 1667 Ω you should still have 30mA which with an RCD to BS EN 61008 should open in 300mS which is 0.3S. Having said that i'm surprised it does not quite conform , but isn't there some cases where the TN time of 0.4S can be used ie all equipotential bonding in place?

 

[Sintra]

Correct.

 

[Ponty Massive]

the rcd is only in the event of an earth fault - in the event of a L-N short the rcd would not trip.
table 41.5 is only for earth faults. The low Zs ensures mcb operation in the event of a short.

Can you explain this one please? By testing Zs we are testing between live and earth. I understand that short between L-N would not cause the RCD to trip but the MCB to go as an over current, but how does the low Zs ensure the mcb operates?

 

[telectrix]

because the fault current caused by a line/earth short will overload the MCB causing it to trip. the lower the Zs, the higher the fault current, so the faster it will trip

 

[malcolmsanford]

As a fault current can be 100's of amps so you would want to clear this, by the protection device tripping as soon as possible. The lower the impedance on a circuit. the Zs, the quicker these currents will rise and therefore the quicker you achieve your tripping.

Not got the BRB to hand of now but a 32amp BS 88 fuse takes hundreds of amps to trip so you can imagine if you have a high impedance on that circuit to achieve that trip time will take longer. That's why the BRB give you tables 41 1 to 4 ( those are the maximum values you then have to calculate or use the "rule of thunb") or the OSG gives you the calcualted values of Zs in appendix 2. Those values are the permited values of the impedance for that protection device to get your 0.4 or 5 sec trips in a TN

Ohms law great stuff

 

[pushrod]

the rcd is only in the event of an earth fault - in the event of a L-N short the rcd would not trip.
table 41.5 is only for earth faults. The low Zs ensures mcb operation in the event of a short.

Can you explain this one please? By testing Zs we are testing between live and earth. I understand that short between L-N would not cause the RCD to trip but the MCB to go as an over current, but how does the low Zs ensure the mcb operates?

Bad grammar really - The last sentence, of my original post, should really have been a separate paragraph as it does not follow directly from the previous sentence, as that one refers to TN systems but table 41.5 refers to TT systems. I take your point.

Malcolm and Telectrix above have explained how it would work with a TN system but with an earth fault on a TT system the rcd will trip but the mcb probs won't. With a dead short on a TT the mcb will trip and it is the low value of R1 + Rn that ensures it happens. Apologies if it caused confusion.

 

[malcolmsanford]

Bad grammar really - The last sentence, of my original post, should really have been a separate paragraph as it does not follow directly from the previous sentence, as that one refers to TN systems but table 41.5 refers to TT systems. I take your point.

Malcolm and Telectrix above have explained how it would work with a TN system but with an earth fault on a TT system the rcd will trip but the mcb probs won't. With a dead short on a TT the mcb will trip and it is the low value of R1 + Rn that ensures it happens. Apologies if it caused confusion.

Interesting post Push, I have only ever done a R1+Rn test on a ring final circuit when your doing the continiuity loop test. Have you ever did the test to prove tripping times ?

I've always done the Zs tests for tripping which is your Zs= Ze+ (R1 + R2) and if it failed you either increased the size of CPC, run an extra CPC or the modern way RCD it.

 

[trebor]

Overcurrent can be caused by , overload, line to neutral fault or live to earth fault

on he line to neutral fault Zs has no relevance

on the line to earth fault, Zs is relevant as it is low impedance of the earth path that will allow enough fault current to flow and operate the device be that a fuse or circuit breaker

taking a 32A type B cb as an example, Ia for this is 160A, to produce 160A we need a Zs of 1.44ohms cold or 1.15 corrected for temps, ie 230/160 = 1.44 x 0.80 = 1.15

 

[pushrod]

Interesting post Push, I have only ever done a R1+Rn test on a ring final circuit when your doing the continiuity loop test. Have you ever did the test to prove tripping times ?

I've always done the Zs tests for tripping which is your Zs= Ze+ (R1 + R2) and if it failed you either increased the size of CPC, run an extra CPC or the modern way RCD it.

Hi Malcolm, i'm very much a beginner myself, but it occurs to me that if you are dealing with a TT system then you need ADS with regard to an earth fault and a dead short. The earth fault is taken care of by the RCD and the fault current in this case will be very small (because of high Ra) not the the 100's of amps you would get on a TN, ie it only needs to be 30mA+. To ensure ADS in the event of a dead short i suppose strictly speaking it is the R1+Rn valuethat is important but if you have continuity and your R1+R2 is okay it would be difficult to imagine that the R1+Rn would not be fine (pauses whilst scratching head and thinking)

 

[malcolmsanford]

It is always good to think Push but I'm not sure your using the R1+Rn correctly, if I'm wrong I apologise immediately.

As we know the RCD works by detecting the balance between the line and neutral. Whilst there is 230v going into the live side of the coil and as long as there is 230v coming out the neutral side we have that balance and non trip.

When a fault occurs, the fault voltage will try and take the path of less resistance which is our earth. As soon as this happens you will get an imbalance on the line and neutral side of the circuit and so on the RCD coil, as that voltage is now going down the CPC cable.

As soon as all this starts in ohms law the voltage leaking to the earth CPC will produce a current or load due to the impedance in that cable, and at 0.03 amps the RCD will trip.

That is our fault protection . What it will not dected is an overload on a healty circuit. That means for instance a motor maybe rated at 10 amps but due to weak winding, or a start winding is wired as the run, and this then draws say 40amps, an RCD will not trip but a cb/fuse will. That is why the RCD can only be used as additional and not sole protection. That is why an RCCB or RCBO can be used as a sole means of protection as it encoporates both types of protection RCD and CB/Fuse.

 

[Ponty Massive]

Just so I know I have understood, would you agree with this statement.

Ze measurement is taken to calculate that required disconnection time can be achieved should a fault to earth occur and provent electric shock to anyone. It has nothing to do with L-N fault.

L-N faults are dealt with by means of MCB preventing overcurrent and protecting cables?

 

[pushrod]

Just so I know I have understood, would you agree with this statement.

Ze measurement is taken to calculate that required disconnection time can be achieved should a fault to earth occur and provent electric shock to anyone. It has nothing to do with L-N fault.

L-N faults are dealt with by means of MCB preventing overcurrent and protecting cables?

Agree

 

[pushrod]

It is always good to think Push but I'm not sure your using the R1+Rn correctly, if I'm wrong I apologise immediately.
I know that the usual context of R1+Rn is when checking continuity on a ring final circuit, but it is this value along with the loop impedance external to the property that will reponsible for determination of the fault current that will flow if a short occurs at the most distant part of the circuit.
As we know the RCD works by detecting the balance between the line and neutral. Whilst there is 230v going into the live side of the coil and as long as there is 230v coming out the neutral side we have that balance and non trip.

When a fault occurs, the fault voltage will try and take the path of less resistance which is our earth. As soon as this happens you will get an imbalance on the line and neutral side of the circuit and so on the RCD coil, as that voltage is now going down the CPC cable. Basically agree with what you are saying here but you are mixing up your current and voltage a bit. The rcd works on current balance with their magnetic fields cancelling each other. When their is a current leakage to earth the magnetic imbalance caused by the difference in currents causes the switch to trip.

As soon as all this starts in ohms law the voltage leaking to the earth CPC will produce a current or load due to the impedance in that cable, and at 0.03 amps the RCD will trip.

That is our fault protection . What it will not dected is an overload on a healty circuit. That means for instance a motor maybe rated at 10 amps but due to weak winding, or a start winding is wired as the run, and this then draws say 40amps, an RCD will not trip but a cb/fuse will. That is why the RCD can only be used as additional and not sole protection. That is why an RCCB or RCBO can be used as a sole means of protection as it encoporates both types of protection RCD and CB/Fuse. Basically agree with this - a large inrush current is typical on motor circuits hence the use of of the other types of mcbs or Gm fuses.

My comments in red - always good to give the brain a work out