sMaximum earth loop impedance for TN systems, and TT system

[hairyass]

I am finding this quite a complicated subject, and I would really welcome your thoughts on this subject.

If I could start with a TT system, according to regs, 411.5.3 states that when using an RCD for fault protection purposes, then the following has to be applied.

RaxIn<=50

This means that if we are using a 30ma rcd then Ra <= 50/0.03 = 1667 ohms earth loop impedance

Obviously they are concerned here with a poor earth connection, hence they have are using the safe voltage limit of 50v.

So for TT systems bs rcd to 4239 with a trip time of <= 0.2 are acceptable, 61008 with a trip time of 0.3 are not.

If i am using a circuit breaker for fault protection, say a 20amp 608686 then for TT i must use the following

ZsaxIa<= Uo 230/100 = 2.3 ohms.

I think what this means is basically if you have a good enough earth loop impedance then it is fine to use an mcb for 0.2 disconnection, instaneous operation being the all important word here, if not then use an rcd, and which is to my mind is treating the circuit then like supplementary bonding given the inclusion of the safe touch voltage.

For a TN system, where an earth is provided, then you must use

ZsaxIa<= Uo for both rcd’s and mcb’s, the regulations are quite clear about this, see 411.4.5

This leads to a pretty important finding for rcd’s the maximum loop impedance of which is 230/0.03 = 7667 ohms, now this is ridiculously high, and if I had an impedance of this, given that on TN systems the Ze should be 0.8 ohms, I would deffo investigate the r1+r2.

I can only conclude that 7667 ohms is a max hyperthetical figure, given there was issues around loop impedance with a Tn system that could be used to ensure instanteous disconnection with an rcd when used for fault protection.

What do you guys think of this 7667 ohms figure???

 

[johnboy6083]

i think that to get a low enough reading to use a normal method of fault protection in a domestic TT setting would be very difficult. bear in mind that it must be stable, and this would require very deepdriven rods, and probably more than one. only then could you have any confidence in the rods.

with shallow rods used by normal sparks in domestic or small commercial settings, they rely a lot on the moisture content of the ground. when the ground is wet, the RA value will be lower than when the ground is dry, or frozen.

the 1667 ohms figure is a hypothetical figure that will enable the RCD to function. bear in mind that RCD's are not immune to failure.
i certainly would never energise an installtion with that reading.

Engineer 54 mentions a lot about us relying on RCDs in TT systems. i actually agree with him. we always try tp provide as much safety backup to an installation as possible, but with RCD's on TT systems there is usuallly only one thing that stands between a safe installation and an unsafe one,. that is an RCD.
ill wait for the flak to come my way, and probably one thanks (ill thankyou in advance engineer 54) lol

 

[Sparky Ninja]

I am finding this quite a complicated subject, and I would really welcome your thoughts on this subject.

If I could start with a TT system, according to regs, 411.5.3 states that when using an RCD for fault protection purposes, then the following has to be applied.

RaxIn<=50

This means that if we are using a 30ma rcd then Ra <= 50/0.03 = 1667 ohms earth loop impedance

Obviously they are concerned here with a poor earth connection, hence they have are using the safe voltage limit of 50v.

Ok

So for TT systems bs rcd to 4239 with a trip time of <= 0.2 are acceptable, 61008 with a trip time of 0.3 are not.

Where is this info from? Are you thinking of the maximum when testing RCD's?

If i am using a circuit breaker for fault protection, say a 20amp 608686 then for TT i must use the following

ZsaxIa<= Uo 230/100 = 2.3 ohms.

I think what this means is basically if you have a good enough earth loop impedance then it is fine to use an mcb for 0.2 disconnection, instaneous operation being the all important word here, if not then use an rcd, and which is to my mind is treating the circuit then like supplementary bonding given the inclusion of the safe touch voltage.

100A is for 0.1 to 5 seconds. For the MCB to operate it will need an effectively sustained Ze to ensure the Zs of 2.3ohms will always be within tolerance (411.5.2 Note 1).

For a TN system, where an earth is provided, then you must use

ZsaxIa<= Uo for both rcd’s and mcb’s, the regulations are quite clear about this, see 411.4.5

This leads to a pretty important finding for rcd’s the maximum loop impedance of which is 230/0.03 = 7667 ohms, now this is ridiculously high, and if I had an impedance of this, given that on TN systems the Ze should be 0.8 ohms, I would deffo investigate the r1+r2.

I can only conclude that 7667 ohms is a max hyperthetical figure, given there was issues around loop impedance with a Tn system that could be used to ensure instanteous disconnection with an rcd when used for fault protection.

What do you guys think of this 7667 ohms figure???

The R1&R2 won't need investigating as it has nothing to do with the value obtained. The R1&R2 contributes towards obtaining a value for Zs, which is used to calculate the fault current level and compare it with the selected overcurrent device to confirm effective disconnection.

The 7667ohms value is representative of earth leakage current (or shock currents). Translated it means that if the Zs goes anywhere over the 7667ohm value, then not enough current will flow to operate the device.

 

[Engineer54]

i think that to get a low enough reading to use a normal method of fault protection in a domestic TT setting would be very difficult. bear in mind that it must be stable, and this would require very deepdriven rods, and probably more than one. only then could you have any confidence in the rods.

with shallow rods used by normal sparks in domestic or small commercial settings, they rely a lot on the moisture content of the ground. when the ground is wet, the RA value will be lower than when the ground is dry, or frozen.


the 1667 ohms figure is a hypothetical figure that will enable the RCD to function. bear in mind that RCD's are not immune to failure.
i certainly would never energise an installtion with that reading.

Engineer 54 mentions a lot about us relying on RCDs in TT systems. i actually agree with him. we always try tp provide as much safety backup to an installation as possible, but with RCD's on TT systems there is usuallly only one thing that stands between a safe installation and an unsafe one,. that is an RCD.
ill wait for the flak to come my way, and probably one thanks (ill thankyou in advance engineer 54) lol

And here it is mate ...and gladly given!!! lol!!!

Flak or no flak, your correct and that's all that matters!! lol!!

At one time, it was difficult to get hold of an earth rod shorter than around 2.8m or smaller than 3/4''. The shorter lengths were generally classified as extension lengths!!!! It wasn't uncommon to see two such standard length rods coupled together either, in suitable soil conditions. What's more, it was more common than not to see 10 ohms in many areas on TT systems around the Essex Area's. Thinking your going to get anything like that figure with a single 1.2m 3/8'' rod is living in cloud cuckoo land. But never mind ....there is always that RCD device(s) to save the day, .....Or maybe Not!!!

Good to see that some electricians here ''Are Listening'' ...lol!!!

 

[Deleted member 26818]

Remember, that if the earth electrode resistance is low enough, and stable, the disconnection times for TN can be used.

 

[johnboy6083]

stable is the operative word, and how would you know that? unless your using deep rods/tapes/matt i dont think you could arrive at that conclusion, as an installation electrician.

not without knowing the resistivity of the soil, and substrate. all of which is beyond me anyway.

of course im open as always to be corrected, but if i had a low reading, i would still not use that low reading to allow me to use normal methods of fault protection. If i had access to several years worth of test results, then maybe i would think differently.

 

[Chr!s]

stable is the operative word, and how would you know that? unless your using deep rods/tapes/matt i dont think you could arrive at that conclusion, as an installation electrician.

not without knowing the resistivity of the soil, and substrate. all of which is beyond me anyway.

of course im open as always to be corrected, but if i had a low reading, i would still not use that low reading to allow me to use normal methods of fault protection. If i had access to several years worth of test results, then maybe i would think differently.

And your not just relying on your Ra, you need to take into the account of the TX Ra, this may be as high as 20 ohms

 

[Engineer54]

On TT systems, you would be relying more on the installations water and gas connections than a distant TX Ra.



Personally, i've never come accross a suppliers distribution TX with an Ra anywhere near as high as 20 ohms. I can however see a case for say, a local pole tap off TX supplying a single installation like say a remote'ish farm or such!!!

 

[hairyass]

So for TT systems bs rcd to 4239 with a trip time of <= 0.2 are acceptable, 61008 with a trip time of 0.3 are not.

My line of thinking here was merely to point out that in order to provide for fault protection disconnections times, an rcd operating at 30ma would have to disconnect < 0.2, and the 4239 bs rcd will do this at a full trip current in less than 200ms or 0.2.

 

[hairyass]

many thanks to all who read this thread, and in particular thanks to those who replied.

 

[Des 56]

Remember, that if the earth electrode resistance is low enough, and stable, the disconnection times for TN can be used.

I have mused a bit on that possible alternative decision
I dont particularly see how that option can be given by the regulations,when the supplier states a maximum of 21 ohms possible on their own earth electrode resistance at the star point, given that it is part of the fault path

 

[IQ Electrical]

So for TT systems bs rcd to 4239 with a trip time of <= 0.2 are acceptable, 61008 with a trip time of 0.3 are not.

My line of thinking here was merely to point out that in order to provide for fault protection disconnections times, an rcd operating at 30ma would have to disconnect < 0.2, and the 4239 bs rcd will do this at a full trip current in less than 200ms or 0.2.

You're getting confused with disconnection times here.

BS 4293 devices were superseded by BS 61008/61009 devices.

If you check out Note 2 at the bottom of the table of disconnection times, all will become clear, the 0.2 s disconnection time is for a fault of negligible impedance, meaning currents far higher than say 30 or 100mA will flow, causing faster operation of the RCD/RCBO device.

Check out Appendix 3 in BS 7671:2008 for disconnection times of 2 X I delta n as an example.