Zs values?

[Guest123]

I would draw peoples attention to 411.4.9

 

[SimpleSimon]

That's not said in the regs. Only regarding TT. But that is because on TT system you are going to get such variable readings from your earth rod they allow you to conform with formula
Ra In > 50v.
reversing the formula
50v/0.03mA= 1667 ohms. That the maximum Zs with an RCD that allows unto the safe touch voltage.
On a TN system you aren't going to get nowhere near that figure.

 

[nick611]

reg 411.5.3 rcd protection 0.2 dissconection any system

 

[Jimmy Boy]

I would draw peoples attention to 411.4.9

Good stuff Lenny so basically your saying 1667 ? for those circuits protected by an RCD/RCBO ? Now I am well confused as the short circuit/overload part of the RCBO surely must meet the Zs in the regs ?

 

[Jimmy Boy]

reg 411.5.3 rcd protection 0.2 dissconection any system

Thanks but doesn't that only apply to RCD's fitted for earth fault protection, not additional protection ? An RCBO on a bathroom is surely additional ?

 

[nick611]

if a tt system ze up to 1667 will disconnect in 0.2 then a tn max 0.80 can,t fail to can it on an rcd

 

[Guest123]

reg 411.5.3 rcd protection 0.2 dissconection any system

I disagree...that is under the heading of TT system.

As I said 411.4.9 leans towards the statement of "where an RCD is used (for whatever reason) to fulfill 411.3.2.2 (where the loop impedance could be too high for the MCB/fuse alone to achieve it) then the values of EFLI in table 41.5 may be applied for RCD's to 61008 & 61009 for final circuits upto 32A.

 

[IQ Electrical]

if a tt system ze up to 1667 will disconnect in 0.2 then a tn max 0.80 can,t fail to can it on an rcd

Nick, I don't think anyone is saying the device wouldn't physically operate, they are looking for a regulation to support the entry of 1667
in the max Zs box.

Also, Ze or more correctly Ra on a TT system is not recommended to be above 200 Ohms for stability (Table 41.5, note 2)

 

[nick611]

lets face it if you can have 200 ohms on a stake c/w rcd then anything lower tncs is no problem.live neutral fault no problem as poses no danger to public.overload on mcb will deal with that

 

[Des 56]

Rcds can and do fail

I personally believe a TN system with a failed Rcd, and Zs readings outside of permitted limits,would appear to be a much greater danger than the TT with a failed Rcd

I did read that the High fault currents on a TN system where the Rcd is designed to operate, at say 30m/amp,it can damage those electronics more readilly than the low fault current TT system

Where the RCd has bit the dust
If someone bridged a live earth fault,the Tn system would allow a massive fault current,where as the TT would be just a tickler

I use the tabulated values for TN systems and the 1667 exclusively for TT

 

[jeremy]

My heads gone a bit "pinotage". I'm going to have to read these again tomorrow!

 

[IQ Electrical]

I did read that the High fault currents on a TN system where the Rcd is designed to operate, at say 30m/amp,it can damage those electronics more readilly than the low fault current TT system

I know we're drifting off-topic here but where did you read this?

 

[Jimmy Boy]

I was speaking to a sparkie other day that said he has had a few RCD's go belly up on a x5 test. I do not and never will place my trust in an RCD of any type to protect people against shock protection, I don't do anywhere the number of installs you guys do, but the amount of failures out of the box frankly frightens me, Hager, MK, RCBO's and RCD's all failed miserably on a number of occasions, technology is a good thing but not always the best thing

 

[SimpleSimon]

As IQ electrical has stated there is no specific reg pointing to this, you'd like to think that the regs would think logically and go with the trend of 'We are trying to push the use of RCDs to cover majority of circuits in a domestic situation, perhaps we should provide a Zs table with RCD protection for TN systems.' have they not done this because they want us to use the different MCB values regardless?
Des could you please explain why it would be more dangerous on a TN? (not questioing just don't understand )

 

[jam2020]

So anyway after all these posts which figure are we putting on the eic?