Bad ZS Readings

[rob196]

Hiya guys, on a TT system I keep getting a very high ZS reading,

ze = 33.1ohms

PFC = 1.8 KA

All r1+r2 reading are good but my Zs reading on all circuits are all coming out at 16.4 - 17.2 ohms.........

Any suggestions ?

 

[batty]

On TT if you are getting ZS readings like this there is nothing wrong you are probably getting lower reading from water pipe or gas but remember as long as there is Rcd protection you can go up to 220 ohms with no further investigation.

 

[sparkytim]

Thats GOOD for a TT

TT means you are using a Rod into the ground for your earth, Zs CAN be as much as 1666.66 ohms and still be ok for 30 m A RCD protected circuits, although a Max 200 ohms is recommended.

TNS supply Maximum Zs values do NOT APPLY TO TT installations

See BRB (BS7671) Pg 50 Reg 411.5 TT Systems and Tabel 41.5

 

[Andy™]

Hiya guys, on a TT system I keep getting a very high ZS reading,ze = 33.1ohms

PFC = 1.8 KA

All r1+r2 reading are good but my Zs reading on all circuits are all coming out at 16.4 - 17.2 ohms.........

Any suggestions ?

assuming 30mA RCD, max Zs is 1667 ohms.

for TT, Ze should be considered unstable if over 200 - your 33.1 is well below this, and a good reading

 

[rob196]

There is 16 circuits and 6 of them are over 32amps ...... all circuits are protected by 30ma RCDS so are ok but what about the circuits that are over 32Amps?????

 

[NozSpark]

There is 16 circuits and 6 of them are over 32amps ...... all circuits are protected by 30ma RCDs so are ok but what about the circuits that are over 32Amps?????

Sorry, don't really know what you're on about here..

As previously said on a TT installation your Ra should be below 200ohms.. the table with max Zs values for TN installations does not apply, however this doesn't mean you can go mad with your R1+R2!

Think about it..

Zs=Ze + (R1+R2)

if your Ze (Ra in this case) is 32ohms then your Zs is going to be something similar...the (R1+R2) component is negligible!

 

[a1spark]

rob196. You seem to be posing two seperate questions here. Presumably you now realise the reasoning behind your Zs readings...so thats question one out of the way. ...You now appear in your second question to be in doubt as the suitability of a 30mA RCd installed as protection on the circuits rated above 32A.

You need to clarify why you are concerned in this respect. Obviously the term 30mA is referring to the value of leakage current the RCD is designed to operate at [max value].Not its rated loading value, or are you concerned over the number of circuits installed on the RCDs...if so then Diversity is the answer to your question........more input required here rob.

 

[rob196]

rob196. You seem to be posing two seperate questions here. Presumably you now realise the reasoning behind your Zs readings...so thats question one out of the way. ...You now appear in your second question to be in doubt as the suitability of a 30mA RCd installed as protection on the circuits rated above 32A.You need to clarify why you are concerned in this respect. Obviously the term 30mA is referring to the value of leakage current the RCD is designed to operate at [max value].Not its rated loading value, or are you concerned over the number of circuits installed on the RCDs...if so then Diversity is the answer to your question........more input required here rob.

Yes I do understand what you guys are saying, just that in Table 41.5 its say Max earth loop impedance (Zs) to ensure RCD operattion in accordance with regulation 411.5.3 for non delayed RCDs to BS EN61008-1 and BS EN 61009-1 for Final circuits not Exceeding 32A ? I'm just not understanding this Bit?????

All circuits are RCD protected so have no worries........

 

[a1spark]

Fair point there Rob. must admit I am not sure of the answer to that one.

Maybe someone can enlighten us both on why this is stated.

 

[a1spark]

Just had another thought on that one rob , and I think the answer lies along the lines of the application of Table 41.5. Is it because circuits above 32A will supply fixed equipment not deemed to require the 0.4 [ 0.2 TT }disconnection time i.e. Additional Direct Contact Protection.[basic ]

In a tt install the RCD will be used in its indirect protection [Fault ] mode cos of the poor Earth path therefore the RCD will still be required to ensure ADS.

,

I may be barking up the wrong tree here ,just trying to work it all out on hoof

In All cases ,however the 30mA rcd protection will be required in all circuits in the Bathroom regardless of what has been said above, and also for cables in walls etc.

 

[rob196]

I thought i was missing something lol.....I hope someone can enlighten us on this....will go and have a :coffee then hope someone will chip in with some sort of answer ray

 

[sparkytim]

Ccts in excess of 32 A and distribution ccts have a max disconnection time of 1 Sec (Reg 411.3.2.4) as apposed to 0.2. So max Zs could be higher then Tabel 41.5

No values appear to be given I would suggest, as thse in Table 41.5 are in practice well above those found in most installations maybe.

 

[a1spark]

I can see where your coming from there Tim, but I have the feeling there is more to it than that otherwise why do they not provide tabulated values for these circuit also ? I must say the regs are less than helpful here, or maybe not immediately clear on the requirements.

 

[JUD]

Theoretically for a maximum disconnection time of 1 sec you would need a fault current of 30mA because the RCD should trip out in ≤ 300mS at it's operating current.

Therefore your theoretical max Zs on circuits over 32A is 7666.66Ω

230

 

[a1spark]

Theoretically for a maximum disconnection time of 1 sec you would need a fault current of 30mA because the RCD should trip out in ≤ 300mS at it's operating current.Therefore your theoretical max Zs on circuits over 32A is 7666.66Ω

230

 

[JUD]

JUD ,would not a 100mA RCD [ or even higher ] still be acceptable in a TT install for circuits greater than 32A which are not in a Special location ?I think this is why table 41.5 is not appropriate in the OPs question.

Appendix 3. Table 3A.

Yes you could. I was just using 30mA as an example as they are the most common.

The values in Table 41.5 are to ensure that touch voltage does not exceed 50V (50V

 

[a1spark]

Rob 196 & Jud.

In the draft Document for the first ammedment to BS 7671 the confusion over this matter appears to be clarified ,as table 41.5 is to be altered so as to now include data for circuits in exess of 32A.

 

[rob196]

Have just seen the amednments and it's now 1533 ohms for final circuits upto 32A and 1667ohms for final circuits over 32A............

 

[JUD]

Are you referring to this:

The alterations seem to have only been made to the > 230V and ≤ 400V column.

The 230V column remains the same.

 

[a1spark]

Yep. i noticed the shift across the table too !