Earth electrode, continuity

[Alessandro]

Hello guys,

I was hoping you could help me understand the following points!!

1.
Supplier says that Ze = 21 ohms. I test the earth electrode resistance (dead method) and get Ra = 72 ohms. Is Zs more or less = 72 + 21 = 93 ohms?
I know from Regulation 411.5.3 that that this should be true (d = delta): Ra * Idn <= 50 V.
I don't understand why, in the same Regulation, note 2 says: "Where Ra is not known, it may be replaced by Zs". I thought Zs should be at least Ze + Ra, am I wrong? Shouldn't I replace Ra with Zs - Ze?

2.

From Regulation 415.1.1: additional protection by RCD is Idn <= 30 mA, and operating time <= 40 ms at 5 times Idn. So my conclusion is that a current of 5 * 30 mA = 150 mA can flow for up to 40 ms. The maximum shock I can get is 150 mA for 40 ms? Isn't that quite a lot?

3.

In the continuity tests I keep reading that I should use a link at the CCU and disconnect bonding conductors, but isn't it a lot quicker and easier to use a connector block and connect, say, line and earth and test this closed loop at sockets / ceiling roses / switches etc? Why go through the trouble of disconnecting all sorts of things to avoid parallel paths?

4.

When the earth fault impedance tester is used for the earth electrode resistance test, I don't understand why my textbook says that it is less accurate. Wouldn't it be a lot more accurate, the values obtained being the actual impedance of the actual loop?
And then it goes on and says that this value is Zs. I thought it would be Ze? I am testing from the incoming line and the earthing conductor disconnected from the MET after all, we are not considering R1 + R2.

Thanks!

 

[Alessandro]

For number 1, it makes sense if I only use Table 41.5. If it's safe with Zs, then it will always be safe with RA.
If Zs = 1667 ohms
and RA = 1667 - 21 = 1646 ohms,
then using Table 41.5:

Zs * Idn <= 50 V
1667 * 30 * 10^-3 = 50.01 V

RA * Idn <= 50 V
1646 * 30 * 10^-3 = 49.38 V

But if I only use Regulation 411.5.3 (ii), i.e.
RA * Idn <= 50 V,
then if I verify it for RA and I get exactly 50 V, then it would not be verified for Zs.
Am I wrong? I know this is all just theoretical, realistically I presume a value like 1667 ohms would never be acceptable, but I was intereseted in the logic behind it.

 

[Richard Burns]

For a TT system the supplier will not specify a value for Ze as this value is determined by the customers earth electrode.
For all practical purposes the value of 21Ω stated in the on site guide may be ignored as irrelevant and pointless. It does not relate to any approved values for an installation of any kind.

If you measure a value for R[SUB]A [/SUB]at 72Ω then this is the resistance of the electrode and the earthing conductor.
Ze would also include the path back to the transformer via line, the suppliers earthing at the transformer and the general mass of earth, however this resistance will be negligible compared to the resistance of the earth rod at the installation.
For example you may measure 72Ω for R[SUB]A[/SUB] and the line etc. resistance may be 0.3Ω, the 0.3Ω would be within the error range of the test instrument so is almost irrelevant.

For the note of replacing R[SUB]A[/SUB] with Zs, I would have said that this should read Ze, however again the sentence above is referring to a final circuit and again the resistance of the final circuit fault loop will be minuscule compared to the electrode resistance.

150mA for 40ms is not a lot of current.
If you consider that for a TNCS or TNS system the fault current can be 1000's of amps for 0.4s (or 400ms) then the values for a TT system are very small.

You would have to specify what you mean by continuity tests. If you are measuring R1+R2 then usually this would only involve the cables for the circuit you are testing. There would be no requirement to remove bonding,etc. as they would not be involved with the test. Therefore I think I am misinterpreting the question here.

When measuring the resistance of the earth electrode if you test using an earth fault loop impedance tester then you are adding the resistance of the rest of the fault path as described above and so the measurement of the resistance of the electrode will be slightly higher than if you were measuring it on its own. This variation is usually insignificant for a TT system and only comes into play when measuring an earth electrode for a supply transformer or such like where much lower and more accurate readings are required.
I would consider it equivalent to Ze and not Zs.

if you were measuring R[SUB]A[/SUB] and getting a value on the limit of the 50V maximum then this would be acceptable, the fact that the full fault path may have a very slightly higher resistance is not part of the regulation. The regulation states that R[SUB]A[/SUB] * Id[SUB]n [/SUB]<= 50V it does not specify a limit for Ze.

 

[davesparks]

For a TT system the supplier will not specify a value for Ze as this value is determined by the customers earth electrode.
For all practical purposes the value of 21Ω stated in the on site guide may be ignored as irrelevant and pointless. It does not relate to any approved values for an installation of any kind.
/QUOTE]

21ohms is the value you will be given if you use the 'by enquiry' method, much like the standard 0.35 and 0.8 values they quote.
This is, I believe, the maximum resistance to earth that may have exist at the substation many years ago. But these days the DNOs will all be working to a much lower value for their substation earth.

 

[telectrix]

as above. that 21 ohms you quoted is a DNO figure for their earthing and totally irrelevant to your TT system.

 

[Alessandro]

If you measure a value for R[SUB]A [/SUB]at 72Ω then this is the resistance of the electrode and the earthing conductor.
Ze would also include the path back to the transformer via line, the suppliers earthing at the transformer and the general mass of earth, however this resistance will be negligible compared to the resistance of the earth rod at the installation.
For example you may measure 72Ω for R[SUB]A[/SUB] and the line etc. resistance may be 0.3Ω, the 0.3Ω would be within the error range of the test instrument so is almost irrelevant.

Ok, now it is a LOT clearer, thank you. But what is the supplier's electrode's resistance? I'm only curious now, in a TN system you would have a nicely paved conductive path for the fault current to travel all the way back to the transformer, but in this case it has to go through two electrodes. If the supplier's electrode is even 2 or 3 ohms, it makes sense that I should add it to RA and only ignore the resistance of the transformer and line.

For the note of replacing R[SUB]A[/SUB] with Zs, I would have said that this should read Ze, however again the sentence above is referring to a final circuit and again the resistance of the final circuit fault loop will be minuscule compared to the electrode resistance.

I think that Regulation 411.5.3 is not referring to a final circuit, because it says that RA may be replaced by Zs. If they mean that R1 + R2 are considered negligible compared to RA and Ze, then I don't understand why City & Guilds 2365 textbook gives, for the purpose of verification, this example.

R1 + R2 = 2 ohms
Ze = 113 ohms
With a Zs of 115, according to Table 41.5, I can use a 30 mA, 100 mA or 300 mA RCD.

150mA for 40ms is not a lot of current.
If you consider that for a TNCS or TNS system the fault current can be 1000's of amps for 0.4s (or 400ms) then the values for a TT system are very small.

Yes, I was just thinking that 150 mA through someone's body for 40 ms was a lot, but I guess it's not. I understand that for the purpose of quick disconnection we need and normally have very high currents in TN installations, and I understand that for additional protection we use RCDs in TN and TT systems. In TT systems we use RCDs also for the purpose of automatic disconnection according to Table 41.1.

You would have to specify what you mean by continuity tests. If you are measuring R1+R2 then usually this would only involve the cables for the circuit you are testing. There would be no requirement to remove bonding,etc. as they would not be involved with the test. Therefore I think I am misinterpreting the question here.

Sorry it wasn't very clear. I was talking about the examples I read in the book where they say that I should remove parallel paths by removing bonding conductors, leave cpc on the earthing terminal and line on the MCB and use a link to connect line and cpc (after zeroing the link on the meter). But I couldn't understand why I can't just remove line and cpc and connect them together.

if you were measuring R[SUB]A[/SUB] and getting a value on the limit of the 50V maximum then this would be acceptable, the fact that the full fault path may have a very slightly higher resistance is not part of the regulation. The regulation states that R[SUB]A[/SUB] * Id[SUB]n [/SUB]<= 50V it does not specify a limit for Ze.

I looks to me that the regulation is also talking about the full path, and uses RA and Zs interchangeably. My problem is that the regulation says two things, unless I've got it completely wrong, but the textbook says the same thing.

NOTE 2: Where RA is not known, it may be replaced by Zs.

So:

1) RA * Idn <= 50 V
OR
2) Zs * Idn <= 50 V

But if RA is known, then I use the first inequality, and if I get 50 V on the first one it looks like it is acceptable, but I am automatically violating the requirements of Table 41.5 because, like you said, Zs will be slightly higher. Am I just being too OCD about it? It was just a matter of logic to me.

But thanks for the answers so far Richard, dave and telectrix, they helped me a lot.

 

[davesparks]

Ra is specifically the resistance of your electrode(s) to earth, the Ze would be this plus the suppliers cable, transformer etc.
The suppliers electrode at the substation is going to have a very low Ra of its own, likely to be less than one ohm.

 

[Richard Burns]

Suppliers electrode resistance is getting beyond BS7671 however it was traditionally a maximum of 10Ω on the HV side and 20Ω on the LV side, now (and before) they try and get lower resistances, where possible, preferably less than 1Ω.
A possible layout can be seen in this example of an OH transformer earthing from the east midlands electricity earthing guidance.


The C&G example uses Ze which is actually different from R[SUB]A[/SUB], it is important to realise they are different but close in value for the purposes of determining safety of disconnection.
R[SUB]A[/SUB] is used in the regulation for the less than 50V limit, it then gets a bit blurred and is subject to interpretation and an intense scrutiny of the wording that may not actually mean anything.
The table of values refers to the above requirements being satisfied if the Zs meets the values in the table, therefore its possible to replace R[SUB]A[/SUB] in the equation with Zs to meet the requirement, but if the R[SUB]A [/SUB]does meet the requirements then the circuit meets the requirements, this is just regulation interpretation. Just go with it if you can.

150mA for 40ms is on the limit of "safe" current for the human body (as is 30mA for 300ms) as shown in the below diagram, these values are on the borderline of transferring to involuntary muscle contraction which can be dangerous.
However if someone is touching something then the fault current experienced would be lower due to the human body resistance.


The description you give sounds more like insulation resistance testing where the earth should remain connected to the earth bar to ensure you test for any casual contacts with other earthed parts.
I may be wrong but I am not sure how parallel paths would come into an R1+R2 measurement unless the containment was metal, such as trunking or SWA. Just checked OSG AMD 3 and there is no mention of disconnecting bonding for R1+R2.
So a bit lost on that one still.