Maximum ZS???

[Flakey]

Hi there,
Im trying to make sure that i understand my maximum ZS value table correctly and would greatly appreciate your help. So for example is the maximum ZS for a given circuit (not the installation as a whole) based on the device rating, type and figure given in the appropriate table?
so a:
bs3871 32a circuit breaker
type b
= maximum Zs of 1.50

Also if i took a r1 + r2 reading and added that to Ze to obviously get Zs would i have to times it by 0.8 as an 80% rule?

Im sorry if the question is long winded or doesnt make any sense at all lol but am grateful for your help


Cheers!!!

 

[Jay Sparks]

If you use the max Zs readings in the OSG you do not need to use 0.8 rule of thumb as it has already been done. The 0.8 only applies to the max zs in the regs book. If your circuit is RCD protected then max Zs is 1667 ohms.

Jay

 

[Flakey]

Thanks jay for your response. I thought i may have to use the 0.8 multiplier with the Zs reading i have taken for a given circuit and so long as the total is below the maximum Zs value for the breaker/type then its acceptable?

Thanks again

 

[Engineer54]

Amazing how every time questions relating to Zs are brought up, there is always someone that is prepared to let an RCD replace the standard automatic disconnection system!! On TN systems, RCD's are classed as ''Additional'' protection NOT the primary disconnection means. (That only applies to TT systems!!) This RCD cure-all philosophy needs urgently addressing, certainly not promoting!!

 

[Jay Sparks]

I wasnt promoting RCDs, just stating what it says in the book. The op was asking about using the rule of thumb to work out max Zs so I thought I would point out the RCD thing aswell. I didnt write the book, I just have to abide by it.

Jay

 

[TPES]

Amazing how every time questions relating to Zs are brought up, there is always someone that is prepared to let an RCD replace the standard automatic disconnection system!! On TN systems, RCD's are classed as ''Additional'' protection NOT the primary disconnection means. (That only applies to TT systems!!) This RCD cure-all philosophy needs urgently addressing, certainly not promoting!!

Do you think there is a problem with RCD's giving a max Zs of 1667 ohms? I understand in a perfect world and in most cases the measured Zs is lower than the rated Zs for the circuit, but by adding that additional protection is surely a good thing right?

 

[ringer]

The problem comes when the measured Zs is over the tabulated values and the installer then ignores this and they then rely on the RCD for primary protection. This is laziness and unprofessional. The correct course of action would be to investigate why there is a high Zs and to correct the problem. The word 'competence' springs to mind.

Sent from my HTC Sensation using Tapatalk

 

[TPES]

Yes I agree, I've doubted this before myself even when the Zs value is just below the max, I've still investigated why it's not what I'd expect given the lower Continuity and Ze readings. There will usually be a difference from the calculated value but it shouldn't be that much more.. As said RCD is additional protection, I've never once fitted an RCD to get over a high Zs reading

 

[Engineer54]

I wasnt promoting RCDs, just stating what it says in the book. The op was asking about using the rule of thumb to work out max Zs so I thought I would point out the RCD thing aswell. I didnt write the book, I just have to abide by it.

Jay

No-one said you wrote the book!! And the OP was asking about understanding Zs table values correctly, not how an RCD will cover his arse if he has an installation with low Zs values...


TPES,

No, why would i have a problem with a device allowing a max Zs of 1667 ohms?? My problem is those that can't or won't be bothered to try and correct a circuit or an installations low Zs values and are quite happy to leave an RCD in place to fulfill regulation requirements. That's not what ''Additional'' protection means at all, that's just the all to common RCD ''cure-all'' philosophy of lazy individuals...