Zs formula

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jaguar1

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What is the maximum Zs for a tt system with a 300ma rcd as i cannot find the formula in the 17th iee regs. Thanks.

 

jaguar1

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Sorry i know the answer is 167 ohms but how do you get the answer

 
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Are we talking Zs or Ze (actually Ra on a TT)?

Zs is dependant on the type and rating of protective device on the circuit in question. Therefore you have not provided sufficient information mate!

However, Ra ( the rod impedance) is more the question. The formula is:

RCD rated tripping current ( in amps) multiplied by Ra must be less than or equal to 50(volts).

ergo 300mA (0.3A) X 167 ohms = 50.1

The max Ra is actually 166.66 recurring.

Now I`ll ask you a question, if I may.

What is this TT installation, that is covered by a 300mA RCD? If it contains socket outlets, intended to be used by unskilled persons, the RCD should be 30mA

HTH

KME

 

patspark

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or 50/0.3 my prefered method because i find it easy to remember the 50v

 

jaguar1

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OK the question is from the mock on line 17th exam question

 

gazzam

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A 300ma Rcd Is Not Used For Personal Protection Its 30ma Max There For The 300ma Would Protect The Whole Installation (fire Protection)and The 30ma For All Things Outside The Equipotential Zone

Sorry If Im Missing The Point

 

patspark

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ah exam question now page 50 table 41.5 i am doing my exam on tuesday

this came up in the mock exam a few of us used the calc the i found the table

 
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50/0.3 =166.66r Is The Way I Was Taught
it`s the same formula m8 - I just wrote it a different way, to emphasise the op`s post.

It doesn`t matter which way you remember it, as long as you know the correlation between the three - that way you can find any one, if given the other two. Same as ohms law.

 
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it`s the same formula m8 - I just wrote it a different way, to emphasise the op`s post.It doesn`t matter which way you remember it, as long as you know the correlation between the three - that way you can find any one, if given the other two. Same as ohms law.
Will you stop using them big words late at night KME!!! X(

I'm gonna have to go and find me dictionary again! :( :( :(

 

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