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Topic Title: TT questions
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Created On: 25 September 2017 01:42 PM
Status: Read Only
Linear : Threading : Single : Branch
 TT questions   - jonny705 - 25 September 2017 01:42 PM  
 TT questions   - leckie - 25 September 2017 02:58 PM  
 TT questions   - Legh - 25 September 2017 03:02 PM  
 TT questions   - leckie - 25 September 2017 03:07 PM  
 TT questions   - Legh - 25 September 2017 04:52 PM  
 TT questions   - leckie - 25 September 2017 05:43 PM  
 TT questions   - jonny705 - 25 September 2017 04:22 PM  
 TT questions   - Legh - 25 September 2017 06:09 PM  
 TT questions   - AJJewsbury - 25 September 2017 06:15 PM  
 TT questions   - mapj1 - 25 September 2017 10:18 PM  
 TT questions   - jonny705 - 26 September 2017 11:16 AM  
 TT questions   - mapj1 - 26 September 2017 01:38 PM  
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 25 September 2017 01:42 PM
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jonny705

Posts: 157
Joined: 26 September 2015

Can anyone shed some light on why my readings on TT system ring final circuit almost are all the same? I have tested some spiked systems and still find it strange to look at the Zs figures and see things like 25.6 or yesterday 55ohms

But since testing these there is virtually no difference between the socket to the point of me thinking its pointless- the reading s for example : 55.3/55.3/55.3/52.4/55.5/52.4/55.3, now I know that no trip tests are a bit vague , but should I just do ZS by calculation via dead testing on test sheet instead - is it likely to me more accurate?

Also I was thinking about the fact the max Zs figures are meaningless as such on a TT system because of the RCD , but what would happen on a new install if the R1/R2 figures exceeded the max allowed ZS figures would it matter?

And finally what or how before RCD's or anything like that existed , how would the fuse ever blow quick enough if the Impedance was so high?
 25 September 2017 02:58 PM
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leckie

Posts: 4447
Joined: 21 November 2008

Originally posted by: jonny705

Can anyone shed some light on why my readings on TT system ring final circuit almost are all the same? I have tested some spiked systems and still find it strange to look at the Zs figures and see things like 25.6 or yesterday 55ohms [IMG][/IMG]


Well if you think about it, if the Ra reading is say 55 ohms and R1+R2 is say 0.5ohms, the R1+R2 reading is insignificant compared to Ra. So your meter would hardly detect a difference between different points.


But since testing these there is virtually no difference between the socket to the point of me thinking its pointless- the reading s for example : 55.3/55.3/55.3/52.4/55.5/52.4/55.3, now I know that no trip tests are a bit vague , but should I just do ZS by calculation via dead testing on test sheet instead - is it likely to me more accurate?

I have always thought that in the TT situation as you describe that the R1+R2 figure is far more important than with a TN system. Testing R1+R2 gives an indication of the condition of the wiring in terms of length of circuit and connections, etc. Imagine you had a ZsDB of 50 ohms and you meter was measuring 51 or 53 ohms, how would you interpret that? It wouldn't make any difference to the operation of the RCD but it might be that the R1+R2 is say 3 ohms when the expected figure might be more like 0.5 ohms. So for me, dead testing is the way to go in TT systems if you want to make a reasonable assessment of the circuit.



Also I was thinking about the fact the max Zs figures are meaningless as such on a TT system because of the RCD , but what would happen on a new install if the R1/R2 figures exceeded the max allowed ZS figures would it matter?


Well say a 30mA RCBO was fitted so the max Zs would be 1667 ohms; Id be a bit worried if I measured anything that high. On a new install you would measure R1+R2 and compare to the expected value based on the design, that's verifying that part of the circuit. You would also need to measure the Ra, Zs at boards and make a judgement on the suitability of that value. One of the things that can give a bit of a squewed measurement is extraneous conductive part that brings in a lower resistance to earth, say a water pipe. Also it might be connected to and adjoining building which is PME so it could bring in a very low value.


And finally what or how before RCD's or anything like that existed , how would the fuse ever blow quick enough if the Impedance was so high?

Well it wouldn't if the value was too high, the time taken would be dependant on the earth fault loop impedance and the characteristics of the particular device. When I started there were voltage operated ELCB's. I am not sure what the measured values of EFLI were required to be when water pipes were relied upon for an earth or direct earthing from a cable sheath as I think it was called back then. One of resident historians will remind us all
 25 September 2017 03:02 PM
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Legh

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Joined: 17 December 2004

Originally posted by: jonny705

Can anyone shed some light on why my readings on TT system ring final circuit almost are all the same? I have tested some spiked systems and still find it strange to look at the Zs figures and see things like 25.6 or yesterday 55ohms


loop testing with a MFI expects to deliver values of less than a couple of Ohms for RFCs, auto-ranging will cause the results to appear the same when attempting to measure a low value on a high scale

But since testing these there is virtually no difference between the socket to the point of me thinking its pointless- the reading s for example : 55.3/55.3/55.3/52.4/55.5/52.4/55.3, now I know that no trip tests are a bit vague , but should I just do ZS by calculation via dead testing on test sheet instead - is it likely to me more accurate?


IMO I would do both.
1. Ze (measured) + R1 +R2 (tabulated) gives a result.
2. Zs by measurement to verify that both cpc and polarity are correct


Also I was thinking about the fact the max Zs figures are meaningless as such on a TT system because of the RCD , but what would happen on a new install if the R1/R2 figures exceeded the max allowed ZS figures would it matter?


The RCD is your primary ADS protection but will require a verified earth connection so a Zs check is necessary.
The use of RCDs for both Basic and fault protection on socket circuits cover situations where R1 +R2 are slightly greater than the max Zs and indeed much greater on TT systems. However, it is regarded by some to be sloppy design not to increase the cpc thereby reducing the overall R1+R2. This also reduces the touch voltage under fault conditions.


And finally what or how before RCD's or anything like that existed , how would the fuse ever blow quick enough if the Impedance was so high?


yes quite common once upon a time to reduce the Ra, See 411.5.2

Legh

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http://www.leghrichardson.co.uk

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 25 September 2017 03:07 PM
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leckie

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Get your 13th edition out Legh and tell us what it said then I cant remember what the max Ze used to be for water pipe or cable sheath earthing was
 25 September 2017 04:52 PM
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Legh

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@Leckie

A bit before my time

Legh

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http://www.leghrichardson.co.uk

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 25 September 2017 05:43 PM
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leckie

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Legh, that's fantastic! Never read a thirteenth before. I did have the yellow/gold coloured 14th edition when I started - the metric one - but I have lost it. If you have a 14th pdf that would be great. I am thinking about starting to go wiring in a retro manner!

Regulation 406 seems to answer the question Jonny
 25 September 2017 04:22 PM
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jonny705

Posts: 157
Joined: 26 September 2015

Thanks for the replies, I knew that there would be a reason for the similar readings I get with my meter. I have often wondered which is the more accurate either a live zs test or a ze/r1/r2 by calculation , they always have been different when you compare them ,But I have noticed that if say you have no rcd or small mcb to trip ,the high current Zs test is always loads more accurate.

The reason I asked about high R1/R2 readings on a new Ring Final was that the last two caravans I checked both about 5 years old and by the same manufacturer had high readings on their respective circuits - one was 2.36 the other 2.78 - both were never touched or messed with, so I was surprised that the manufacture would rely on the RCD to deal with it on new product.

The regulation regarding high Zs was asked at my last exam - extra supplementary bonding /bigger cpc's s or an rcd, but I assume years ago then, it was achieved by the water or gas pipes etc in the ground?

But what would happen if you had a rural property with no, say water or pipes but electricity , and no rcd type devices existed?
 25 September 2017 06:09 PM
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Legh

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But what would happen if you had a rural property with no, say water or pipes but electricity , and no rcd type devices existed?


Well, lets look at the earth fault path

1. suppliers transformer neutral / earth connection to an electrode /spike or plate

2. ground resistance

3. consumers earth electrode

4. installation MET

5. Individual final circuit cpc

6. Point of fault

7. live conductor/phase

8. circuit fuse / mcb

9. live tails

10. Service fuse

11. Suppliers phase conductor

12. Suppliers Transformer winding

The electrical paths across the ground between the electrode entry points are regarded as infinite so the bottle necks will be around the electrodes.

Several ways to decrease the bottleneck resistances are:

1. Increase the csa of the electrodes

2. increase the number of electrodes

3. increase the depth of each electrode

4. Use lengths of conductive buried horizontal copper tape surrounding buildings and electrode

5. Use the structural steel of the building (if applicable)

6. Supplement the chemical activity of the soil

Lightening protection companies use electrical conductive cement

I read somewhere a maximum of 0.5 Ohms could be achieved and indeed once I managed to get a resistance measurement of 0.45 Ohms on an earthing electrode without the use of a RCD / VOELB.

Legh

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http://www.leghrichardson.co.uk

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 25 September 2017 06:15 PM
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AJJewsbury

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And finally what or how before RCD's or anything like that existed , how would the fuse ever blow quick enough if the Impedance was so high?

The "alternative method" was to limit the resistance only of R2 - knowing what current would open the protective device in the required time (typically 5s) you'd then choose a limit for R2 based on that current and an acceptable touch voltage (perhaps 50V). There would be no guarantee that the required fault current would flow - if it did the required disconnection time would be satisfied, but if it didn't and so the disconnection time would be longer (or never happen) then the voltage generated between the MET and the fault (i.e. along R2) would be below the chosen limit (50V) so no significant shock risk would exists within the premises.

Even earlier versions of the regs didn't even bother with the calculation - just specifying a maximum resistance between exposed- (or extraneous) conductive parts within the premises - usually 1 Ohm (but given the size of fuses at the time, the effect was similar).

(The approach behind the alternative method is still mentioned in the 17th, if you look hard enough).

- Andy.
 25 September 2017 10:18 PM
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mapj1

Posts: 9704
Joined: 22 July 2004

The rule 406 requires "fault current" (what we would now call PSSC) to be 3 times fuse rating, would have been easier to meet in an installation with power being no more than a few 5 A socket circuits. ( a 15A fault current would be achieved with almost 15 ohms, perhaps a single 4 ft rod in clay, and well in reach of a few meres of lead water pipe in a loamy soil at least)
Once you have 30A rings and cooker circuits would needs more like 90A, so about 3 ohms. This is a much harder target needing the sort of mesh and multiple electrodes you may find at a substation, less likely to be met for a farmhouse. Hence the adoption of VOELCBs.

The 3 times rule gives results that are a little way off the five seconds in the current standards, but certainly the same league, so in effect when protection is against fire, and not for safety of life which needs 'sub-heartbeat' speeds, similar times and thinking are still being applied today.


I am told, by those older and greyer than I, (and if honest, I'm going more than a bit grey...) That before the loop tester, came the test lamp and if it lit between L and E, then it was OK for lighting. If it stayed lit with a larger load added between L and E, then the earthing was OK for power too. The test device being a load of a kW or so wired to a plug but L-E, instead of L-N, and a push button.
Now actually the modern loop tester does a broadly similar thing, rather more scientifically, but adds some useful safety features such as stopping the test if the voltage collapses by more than 50, (as that probably means open earth and 50V on exposed parts), and live test duration limiting to the same sort of time as ADS for protection of life, so 0.2-0.4 seconds (that 'heartbeat' time I mentioned above.)
However, allowing current to flow and looking at the voltage droop is still the basic principle.

-------------------------
regards Mike


Edited: 25 September 2017 at 10:58 PM by mapj1
 26 September 2017 11:16 AM
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jonny705

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Joined: 26 September 2015

Ok thanks for all that , I have read some of the old regulations book, and like you say it's still saying use a earth leakage breaker.

But in section 409, it says that an acceptable way is the outer sheath supply cable or a 5th wire in an O/H supply which is basically an TNS system isn't it ?

It then goes on to say if conditions are met by (I think the supply authority) you can PME it.

It also says it's prudent to still have an additional spike or earthed pipe connected to the above PME system.
Isn't this something that is now coming back in the 18th?- and if so why would it be dropped then come back?

Other than the potential loss of the neutral conductor if damaged problem on a PME system, I still cannot get my head round why it isn't abetter way (due to how low the Ze will be ) than a TT system.
Especially when you take the advice from that old reg's book and effectively have a TNCS, but with a TT spike back up as such.

I have read that the DNO earth a PME supply at various point along its distribution, but if all TT systems years ago had been converted to PME , but still retained their spikes/water pipes etc as well surely that be getting the best of both words so to speak ?

And a final question which I know has been asked many times I bet , is why did the bonding get dropped on a Kitchen sink, it has never made sense to me.
 26 September 2017 01:38 PM
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mapj1

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If you are holding a faulty kettle with a live body, and you reach for the taps, would you rather it was earthed, or fed by plastic pipes and isolated ?
Depends how many of each sort of accident you get.
Ideally you'd rather the ADS turned off the faulty kettle, of course, but electricity is very safe so to get any improvement it is all about multiple unlikely events conspiring .
There are many areas in the earth it or isolate it ? debate where the best answer is not clear cut.

On the PME /TT thing. Yoyu may ask if really large fault currents better or worse than small ones, and with RCDs you can disconnect more or less equally fast in either case, which does more damage ?

-------------------------
regards Mike
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