Zs on TT

[Hawkmoon]

What is the point in carrying out zs on TT system other than verifying a loop.
We got an Ra of 14.8 but when all bonding is in place it comes down to 1.9 ohms for most circuits zs results. I just don't see the point if rcd protected .

Assuming this is not a wind up...

Forget the fact this is TT and rethink what a Zs test is for

 

[Hawkmoon]

I'm just lost for words!!

An Ra of 14 ohms and Zs of 1.9 ohms, .....Yep why bother testing??

On reading this - coffee exited my nose

 

[i=p/u]

On a 6A lighting circuit max zs is 6 odd ohms to disconnect a fault within specified time . On this TT system with 100mA rcd, our Ze is14 ohms soothe breaker won't trip within time, so relying in rcd.. Oh my god

 

[Hawkmoon]

On a 6A lighting circuit max zs is 6 odd ohms to disconnect a fault within specified time . On this TT system with 100mA rcd, our Ze is14 ohms soothe breaker won't trip within time, so relying in rcd.. Oh my god

I hope the 'Oh my god' signifies the end of your blonde moment!

 

[i=p/u]

I should of started work 30 mins ago... I. Beg belief

 

[i=p/u]

Tell me I'm not having blonde moment.... My hairs gone since this thread started.

 

[GLENNSPARK]

jesus wept....

 

[Zebra Stripe]

I think this is a good question and one that deserves better treatment than most of the answers given so far.

So IMO a value obtained via an earth loop impedance test on a domestic TT installation, is pretty much academic when, as in most cases, an RCD has to be used for earth fault protection.

A measured Zs value usually bears very little resemblance to the equation Zs = Ze + (R1+R2), with Zs values obtained on many TT systems falling within the TN range due to parallel paths etc. So what significance does a Zs measurement have in a domestic TT system when an RCD has to be used for fault protection, apart from confirming the earth loop path is still in place once the installation is “live” and that the installation has good parallel earth paths?

The important bit is obtaining a correctly tested Ra/Ze value and confirming that the equation Ra x I delta n is < or equal to 50V, and that the final circuit R1+R2 values are ok. Although if an Ra/Ze test can’t be carried out, you could use a measured Zs instead of Ra/Ze in the 50V equation above.

 

[i=p/u]

And if an rcd is used , in this case a 100mA in doesn't mater if your zs is 356 ohms

 

[Hawkmoon]

And if an rcd is used , in this case a 100mA in doesn't mater if your zs is 356 ohms

How about if it's 3560 ohms?

 

[MDJ]

And if an rcd is used , in this case a 100mA in doesn't mater if your zs is 356 ohms

Hey fella, have a read of note 2 411.5.2, that will help you and may clear things up. Regards.

 

[MDJ]

How about if it's 3560 ohms?

it would exceed the maximum earth fault loop impedance as table 41.5 for a 100MA which is 500 Ohms, that said any reading above 200 ohms should be considered unstable 542.2.4

 

[MDJ]

No glen not have a good go, no no no , support what I am saying.

if you have ze of 14 ohms what is the point of doing zs test , as you know your readings are to high for protective device already.


Brains just ain't your friend last two days guys , AAA?

Note 2 again?

 

[Zebra Stripe]

I think your missing the point of the question, or perhaps its me.
I thought i=p/u was asking what is the significance of a measured value of Zs on a final circuit in a TT installation. What is the reason for doing it in this instance and is it really necessary if earth fault protection is provided by an RCD and cannot be met within the stated time by an MCB.

 

[MDJ]

The answer to ensure the reading is under 1667 or 500 Ohms depending on what RCD is in place. although the RA may be within parameters if the ZS is lets say for argument sake 1000 ohms and the RCD 100 ma then the spark needs to know this?

 

[Zebra Stripe]

But, this can surely be determined via an acurate Ra measurement and an accurate R1+R2 measurement for each final circuit, at least I think this is what he's getting at.

 

[MDJ]

True, however it is just as simple to ZS the circuit as well, that said note 2 of 411.5.1 says "where an RCD is used for for fault protection the circuit should also incorporate an overcurrent protective device in accordance with chapter 43" so if your selecting an overcurrent device then you may well test ZS to see if it would operate under fault conditions, the RCD could fail, if so you are relying on the other protective device. and if that didn't convince you then the schedule of results pages in our EICs or EICRs still have a column for ZS readings, I don't like calculating ZS, I like to test for it myself.

 

[Zebra Stripe]

Agreed, I like to test it myself too, but an Ra plus R1+R2 of each final circuit will give a more accurate/stable assessment of the earth fault loop impedance and would not include parallel paths which may be removed at some point.
I've had Zs values on TT installs that fall easily within the TN bracket and would suggest that an RCD would not be required to meet disconnection times, but if these parallel paths were to be removed then obviously the tiny earth fault currents would not disconnect at any time. So how significant is a measured Zs in this instance?

 

[MDJ]

Of course we also need to remember the regs regarding testing and inspecting circuits and verification of results before handing over a certificate to the customer, another reason to test ZS IMO

 

[Zebra Stripe]

But Im pretty sure there is no requirement to actually test Zs, it can also be calculated using a measured Ze or Ra and a measured R1+R2 and still be compliant to BS7671.