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Zs ohms with maximum rcd ohms

Discuss Zs ohms with maximum rcd ohms in the Electrician Courses : Electrical Quals area at ElectriciansForums.net

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Wilson12

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If the maximum zs is okay up until 1667 ohms protected by a rcd why do we care about maximum disconnection times for the mcbs e.g why do we test and it's important to meet 1.37 or 1.1 zs on a 32 amp circuit
 
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Wilson12 said:
If the maximum zs is okay up until 1667 ohms protected by a rcd why do we care about maximum disconnection times for the mcbs e.g why do we test and it's important to meet 1.37 or 1.1 zs on a 32 amp circuit
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because we should aim for fault protection to be provided by the OCPD, not reliant on a RCD.
 
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Wilson12 said:
If the maximum zs is okay up until 1667 ohms protected by a rcd why do we care about maximum disconnection times for the mcbs e.g why do we test and it's important to meet 1.37 or 1.1 zs on a 32 amp circuit
Click to expand...

Not all circuits are protected by RCD.
The regulations don't allow RCDs to be used as the sole means of protection, they are only additional protection. So you need to ensure that protection is provided by the ocpd.
 
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No this is just me wondering.im doing a eicr around the house and worrying about high zs readings looking for loose connections and I'm wondering why is the regulations even there to abide by.i can't think why it matters if the readings are over the maximum zs readings as theirs a rcd split load preotecting them .
 
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Wilson12 said:
If it's a split load board is there any reason to worry if the zs readings are higher then the mcbs specified
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Well if it's a TN type earthing system , you got to wonder why the zs readings are high?
You will of course be covered by a dual rcd consumer unit providing the necessary fault protection for your 0.4 or 5 second disconnection times
 
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Ian1981 said:
Well if it's a TN type earthing system , you got to wonder why the zs readings are high?
You will of course be covered by a dual rcd consumer unit providing the necessary fault protection for your 0.4 or 5 second disconnection times
Click to expand...
not the point. using the RCD as fault protection is at best a fudge, and at worst, dangerous. a circuit that should give a Zs reading of, say, sub 1.0 ohms and has a reading of 4.0 ohms may have a bad connection that could lead to arcing and a possible fire risk.
 
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telectrix said:
not the point. using the RCD as fault protection is at best a fudge, and at worst, dangerous. a circuit that should give a Zs reading of, say, sub 1.0 ohms and has a reading of 4.0 ohms may have a bad connection that could lead to arcing and a possible fire risk.
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Not disputing that , just that there will be fault protection afforded by the rcd, I did say you've got to wonder why you have a high zs reading on especially a TN system
 
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I would look at it from the point of view of considering a similar situation with the line and neutral loop.
Even though we do not test for this is you have a high line neutral loop impedance then the MCB may not disconnect in time to protect the cable, measuring the Zs at least measures the line side of the loop.
If the Zs is high this may be from circuit design limitations or from errors in installation, if it is errors then you can make attempts to correct, if it is from design limitations then it gives you the chance to assess whether the circuit will comply with the requirements of 434.5 and meet t= (k²S²)/I² (which is just a rearrangement of the adiabatic equation to allow you to determine maximum disconnection times).
 
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Use the alternative Zs method a bypass the RCD! You may see the results drop using this method on hi current test.

Wilson12 said:
No this is just me wondering.im doing a eicr around the house and worrying about high zs readings looking for loose connections and I'm wondering why is the regulations even there to abide by.i can't think why it matters if the readings are over the maximum zs readings as theirs a rcd split load preotecting them .
Click to expand...
 
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Just an expansion on this.

Do you allow any tolerance on the max recorded Zs? I.e. 0.61 ohms on when max zs is .55 this is based on the 80% rule and not the maximum permitted zs
 
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