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Discuss maximum Zs in the Periodic Inspection Reporting & Certification area at ElectriciansForums.net

EddieB

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Table 41.3 in the regs states Maximum Zs for type B breaker 60898 is 1.44 ohms, the domestic electrical installation cert asks what is the Max Zs permitted. Do you insert 1.44 or the lower figure that is mentioned in the on site guide after fault temprature etc has been allowed for. I've been inserting what the table tells me 1.44ohms
 
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S

spark1

you ve been entering the correct figure Eddie.




Regards Spark1.
 
S

Spudnik

Agreed. Maximum Zs according to 7671.

Your reading should be within 80% of this value at normal operating temp.
 
Just to chuck a spanner in. If it's a domestic situation the max Zs is meaningless if it is under 1667 ohms because the 30mA RCD will kick in 1st


Devils Advocate
 
S

Spudnik

An RCD can be used where disconnection times CANNOT be met.

Its not a get out clause.:)
 
An RCD can be used where disconnection times CANNOT be met.

Its not a get out clause.:)

Never said it was.......... But what i am saying is....... Prior to the 17th if like you say disconnection times could not be met.

We should fit a 30mA RCD

Now with the 17th in a domestic situation we fit 30mA RCD as standard, so technically it shouldn't matter as long as it's below 1667 ohms.

I'm just playing Devils Advocate.
 
S

Spudnik

Never said it was.......... But what i am saying is....... Prior to the 17th if like you say disconnection times could not be met.

We should fit a 30mA RCD

Now with the 17th in a domestic situation we fit 30mA RCD as standard, so technically it shouldn't matter as long as it's below 1667 ohms.

I'm just playing Devils Advocate.
Fair do's! :D
 
S

spark1

Would it not be correct to state that the new requirement for rcds in b.s. 7671 2008 is for Basic protection against shock ,by careless users .Granted ,the rcd is also providing a degree of Fault protection and would allow for a theoretical allowable Zs of 1667 ohms.....however the circuit should have been designed with the Protective device providing the primary function of Basic protection, and as such it is the c.b value of max Zs and not the rcd value of max Zs which should be entered into the data box.


Regards..............spark1
 
S

Spudnik

Would it not be correct to state that the new requirement for rcds in b.s. 7671 2008 is for Basic protection against shock ,by careless users .Granted ,the rcd is also providing a degree of Fault protection and would allow for a theoretical allowable Zs of 1667 ohms.....however the circuit should have been designed with the Protective device providing the primary function of Basic protection, and as such it is the c.b value of max Zs and not the rcd value of max Zs which should be entered into the data box.


Regards..............spark1
Yes you are correct...
 
S

spark1

Cheers Jason...........I meant to insert an image reply but not sure how to..........any help available ?

Cheers Jason ! i intended to reply with an inserted image ,but being new to posting here i am not sure how to...........any tips ?

oops double posted also........sorry !
 
Last edited by a moderator:
W

WayneL

Can't believe what I'm reading here, chaps

A 30 mA Rcd is additional protection and only reacts to an Earth Fault

You can't just bang a 30 mA Rcd in and forget about maximum Zs

What about Overload or Short Circuit??

1667 is max Zs for a TT system and even then you wouldn't leave it
anywhere near that high because you wouldn't meet you circuit breaker
disconnection times!

Come on!
 
W

Wotsit

Can't believe what I'm reading here, chaps

A 30 mA Rcd is additional protection and only reacts to an Earth Fault

You can't just bang a 30 mA Rcd in and forget about maximum Zs

What about Overload or Short Circuit??

1667 is max Zs for a TT system and even then you wouldn't leave it
anywhere near that high because you wouldn't meet you circuit breaker
disconnection times!

Come on!
I agree 411.5.3 states 1667 is the max impedance to ensure rcd opperation note 2 asks for an impedance as low as possible and not exceding 200 ohms
 
W

WayneL

Just re-read my post and kind of confused myself.

What I think I'm trying to say is this

You should try get your Zs as low as possible (certainly within the limits

given for the breaker that your using), so that the breaker will trip under

any fault condition, thereby protecting the circuit.

A 30mA Rcd is additional protection against electric shock
and will therefore trip at 5I in 0.04 seconds -
but not all circuits are suited to Rcd protection, are they,
so you have to get the Zs to the limits stated

I think Ive got it right now

Cheers

Wayne
 
S

spark1

wayne & wotsit............agreed ! ...no argument there.

As I said.........but put another way !


Regards........spark1...........
 
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