Zs ohms with maximum rcd ohms

[Wilson12]

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

 

[Octopus]

Is this a college question?

If so what do you think the answer is?

 

[telectrix]

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

because we should aim for fault protection to be provided by the OCPD, not reliant on a RCD.

 

[davesparks]

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

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.

 

[Ian1981]

Actually in a tt system for example, the rcd in question whether be 100mA or 30mA is need for fault protection as well
The mcb is then only providing overload protection to the circuit or short circuit protection

 

[Wilson12]

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 .

 

[Wilson12]

If it's a split load board is there any reason to worry if the zs readings are higher then the mcbs specified

 

[Ian1981]

If it's a split load board is there any reason to worry if the zs readings are higher then the mcbs specified

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

 

[telectrix]

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

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.

 

[Ian1981]

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.

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

 

[Wilson12]

Thanks

 

[Richard Burns]

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).

 

[Simonslimline]

Use the alternative Zs method a bypass the RCD! You may see the results drop using this method on hi current test.

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 .

 

[Richard Burns]

Just thought I would post this here before Tel!

 

[cprfenom]

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

 

[Wilko]

The 80% factor is a good estimate of copper resistance at 70C when remeasured at 20C. So it's not a tolerance.
The BS7671 tabulated values will have the conductor temperature stated, and it's often 70C. We tend to test and measure when a circuit is at room temperature and so we multiply by 0.8 to get an equivalent value.

 

[Richard Burns]

The 80% factor is a good estimate of copper resistance at 70C when remeasured at 20C. So it's not a tolerance.
The BS7671 tabulated values will have the conductor temperature stated, and it's often 70C. We tend to test and measure when a circuit is at room temperature and so we multiply by 0.8 to get an equivalent value.

I think I have probably misinterpreted your words, but the proximity of the 80% appears to refer to correcting a measured resistance to a 70°C resistance.
Whereas it is the 70°C resistance that is multiplied by 0.8 to give an value close to the expected measured resistance at 20°C.
Or alternatively the resistance measured at 20°C is multiplied by 1.2 to give the expected resistance at 70°C

 

[Wilko]

... it is the 70°C resistance that is multiplied by 0.8 to give an value close to the expected measured resistance at 20°C...

Thanks - that's what I meant to say