Zs values?

[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?

 

[nick611]

rcd get stuck if you dont test them regular. i,ve been bit by one myself.but the fact remains 1667 ze 0.2 disconnection.who are we to judge

 

[Jimmy Boy]

Well I am just waiting for the first test case for a fatality due to an RCD not operating and saving a life, I fitted a Power breaker RCD/FCU the other day, guess what >400ms over range about as much use as a fridge to an Eskimo, if a domestic inspect is 10 Years on a new install with the lower lethal level of 50ma what's the chances of it operating in <30ma 9 years time ?..None.
I retested mine at home today all come in at under <19ms @ x5

 

[Des 56]

Energy let through limitation
The supply is 50Hz, i.e., 1 cycle every 20ms, even if the RCD can disconnect at 40ms, you'll get 2 cycles of the full fault current
A typical fault can be in hundreds of amps

This from another source

The surge current refers to the peak current an RCD is designed to withstand using a test impulse of specified characteristics ( an 8/20 µs impulse, named after the time constants of the rise and fall of current).
The IEC 61008 and IEC 61009 standards impose the use of a 0.5 µs/ 100 kHz damped oscillator wave (ring wave) to test the ability of residual current protection devices to withstand operational discharges with a peak current equal to 200 A.


These are others quotes and I just take note of them, bright ideas are rare coming out of my head unfortunately
I read those as saying to me.an Rcd faulting on a Tn system is liable to damage


Des could you please explain why it would be more dangerous on a TN?

low Zs of the TN would have virtually just the persons resistance as the limiting factor,kallowing a massive current flow
High resistance TT rod would limit the current flow
Both could kill,but the Tn would do a better job of it

 

[Geoffsd]

To be pedantic, if that's the right word, the actual value is 1666.66 recurring. Therefore, to be on the 'safe' side we should be using 1666? 1667 gives a voltage of 50.01 - far too high!

Seriously though, I can see the logic on both sides of the argument. However, the maximum Zs is to protect the cpc. So, if we accept that the RCD may fail, should we still not use the maximum Zs for the MCB (or OC part of an RCBO) on a TN system just in case?

 

[malcolmsanford]

1667 is for TT systems.
On a TNS system the mcb or rcbo is your method of complying with 411.3.2.2 & you use table 41.3
The rcd is for additional protection so 1667 does not come into it.

Table 41.5 is for TT systems.

It is alo for TN system Reg 411.4.9 comes under TN. Under this reg table 41.5 can be used in a TN

 

[Guest123]

It is alo for TN system Reg 411.4.9 comes under TN. Under this reg table 41.5 can be used in a TN

At least someone acknowledges that.

I posted it twice earlier in the thread to stimulate responses but got no takers.

 

[malcolmsanford]

A lot of people don't even realise this reg is under the TN section. When the 7671-2008 came up as DPC I wrote to the IET with several questions and this was one. Why start the TN section back on page 47 under 411.4 and end it on a TT page 411.4.9. pg 50

Told in short shrift that 411.4.9 is obviously still under TN as TT starts the 411.5 section!!! hush my mouth

 

[telectrix]

Well I am just waiting for the first test case for a fatality due to an RCD not operating and saving a life, I fitted a Power breaker RCD/FCU the other day, guess what >400ms over range about as much use as a fridge to an Eskimo, if a domestic inspect is 10 Years on a new install with the lower lethal level of 50ma what's the chances of it operating in <30ma 9 years time ?..None.
I retested mine at home today all come in at under <19ms @ x5

RCD failing to trip? that's as unlikely to happen as the "unsinkable " Titanic sinking, or the Hindenberg catching fire! Personally , i consider RCDs to be secondary protection , albeit 95% reliable

 

[Smnspark]

does any one have Max ZS for bs88 315 amp fuse

 

[malcolmsanford]

Something that size you will have to go to the manufacturer and get their data on it. Is it a Bussman?

 

[Deleted member 38070]

b=5x c=10x d=20x
zs = nominal voltage/Ia (current causing automatic disconnection) x0.8
examples of making disconnection times with various mcb types
eg b32
b x 5=160 230/160=1.43 x 0.8 = zs max of 1.15ohms
eg c20
c x 10=200 230/200=1.15 x0.8 = zs max of 0.92ohms
eg d16
d x 16=320 230/320=0.71 x 0.8 = zs max of 0.57ohms

c and d type breakers are used for circuits with high inrush currents

ive gotta get out more!

 

[Smnspark]

think its mem so old but hey ill put a new one in then i can ring them up cheers

 

[telectrix]

somewhere around the 0.12 area IMO.

 

[SimpleSimon]

b=5x c=10x d=20x
zs = nominal voltage/Ia (current causing automatic disconnection) x0.8
examples of making disconnection times with various mcb types
eg b32
b x 5=160 230/160=1.43 x 0.8 = zs max of 1.15ohms
eg c20
c x 10=200 230/200=1.15 x0.8 = zs max of 0.92ohms
eg d16
d x 16=320 230/320=0.71 x 0.8 = zs max of 0.57ohms

c and d type breakers are used for circuits with high inrush currents

ive gotta get out more!

That's a great formula but would it still apply to bs88 fuses?

 

[Deleted member 38070]

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

 

[JUD]

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

What gives you that idea??

Zs = Uo ÷ Ia is basic ohms law (R = V ÷ I).

Zs = Earth fault loop impedance -- Uo = Nominal voltage -- Ia = Current causing operation of the protective device in the required time

The 0.8 is a rule of thumb. The max Zs values in BS 7671 are at a cable operating temperature of 70°C but the cable won't be at that temperature when you take a Zs measurement, so you multiply the values in the BRB by 0.8 to account for the reduced temperature of the cable.

Ia is taken from the time/current characteristics graphs/tables in Appendix 3 of the BRB.

That's a great formula but would it still apply to bs88 fuses?

So yes, it does apply to BS 88 fuses. It's just easier to work out for 60898 and 61009 breakers because it's just 5 x In (type B), 10 x In (type C) and 20 x In (type D).

 

[Geoffsd]

Soooo - Smiffy is right then.

So yes, it does apply to BS 88 fuses.

Obviously, Ohm's law applies to everything but -

It's just easier to work out for 60898 and 61009 breakers because it's just 5 x In (type B), 10 x In (type C) and 20 x In (type D).

...there is no one value which applies throughout the amperage range of BS88s to calculate the maximum Zs for each.

 

[JUD]

Soooo - Smiffy is right then.
Obviously, Ohm's law applies to everything but -...there is no one value which applies throughout the amperage range of BS88s to calculate the maximum Zs for each.

No. What Smiffy said is that Zs = (Uo ÷ Ia) x 0.8 cannot be used for anything but 60898 and 61009 breakers.

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

Uo ÷ Ia is how max Zs values are calculated no matter what the protective device.

Have a look at the time/current graph for BS 88 fuses on page 247 of the BRB. Let's say you have a 32A fuse.

For a disconnection time of 0.4s you need a fault current of 220A. Zs = Uo ÷ Ia (230V ÷ 220A = 1.04Ω). Now compare this with the value in Table 41.2 on page 48.

You then multiply this value by 0.8 to account for the reduced temperature of the cable -- 1.04 x 0.8 = 0.83Ω. This is your maximum measured value.

 

[Geoffsd]

Yes, I know but smiffy's thread is about the fact that you don't need the graphs for 60898s because the relationship between Ia and In is linear,

i.e. 5, 10, 20 for B, C, D respectively.

This is not the case with BS88s and, indeed, other fuses.

 

[JUD]

Yes, I know but smiffy's thread is about the fact that you don't need the graphs for 60898s because the relationship between Ia and In is linear,

i.e. 5, 10, 20 for B, C, D respectively.

This is not the case with BS88s and, indeed, other fuses.

He didn't say anything about not needing the graphs.

What he said in post #75 was the formula for calculating max Zs "only works for B C D mcbs 60898 rcbos 61009". This simply isn't true.

 

[Geoffsd]

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

Jud - you are strictly correct but post #75 was in reply to another post and I, at least, think it quite obvious that he is referring to his discovery/realisation that the 5, 10, 20 only works with 60898s.

Also, as you can see he does refer to 'without looking it up' or as I put it 'needing the graphs'.

I just got the impression that you were 'raining on his parade' when he wasn't really wrong if read in conjunction with his previous post.


Edit -substituted I for you

 

[JUD]

Jud - you are strictly correct but post #75 was in reply to another post and I, at least, think it quite obvious that he is referring to his discovery/realisation that the 5, 10, 20 only works with 60898s.

Also, as you can see he does refer to 'without looking it up' or as I put it 'needing the graphs'.

I just got the impression that you were 'raining on his parade' when he wasn't really wrong if read in conjunction with his previous post.


Edit -substituted I for you

I didn't intend to rain on his parade. His previous post regarding 60898 and 61009 breakers was bang on the money.

I was replying soley to post #75, which is why I quoted it. SimpleSimon asked if the formula also applied to BS 88 fuses, to which Smiffy replied...

max Zs= U0/Ia (x0.8) only works for B C D mcbs 60898 rcbos 61009. (table 41.3) makes life easier to work out max zs without looking it up... fuses i look up. ;-)

 

[Geoffsd]

As I said - "you are strictly correct".

t f b. :seeya:

 

[Dan]

Just bumping some threads in the Electrical Wiring Theories and Regulations forum category here on our Electricians Forums. If this specific topic isn't current, just ignore it, it'll drop off the list in no time. However, if it's something you'd like to contribute to, feel free to reply and bring it back into current discussion.