Testing tt system

[KNIPEX]

Hi guys did a perodic test on a house today, was a tt system, all readings were coming back 493 ohms to 503 ohms which seems a little high to me.

however max zs on tt systems on various mcb is not in the regs book, so what do you guys put in for you max zs???

do the above reading seem high to you??

also the board had an rcd as a main switch, but was not time delay, think im right in saying that a tt system has to have a time delay rcd,

i was trying to explain this to the customer but he wasnt having it, he had an rcd and thats it, how do you people explain to a customer about a normal and time delay rcd?? any easy ways?? i tried my best today.

would you fail the installation from the above?? and put in unsatisfactory???

thanks.

 

[Spudnik]

You Zs and Ze readings, although very high are within the 1667 limit for TT.

They should be around 200 but this can be unstable, so ideally around 100.

 

[Mr Mark Sparks]

agree
regs say 200 nic say 100 somewhere in the middle would do

 

[Guest123]

however max zs on tt systems on various mcb is not in the regs book, so what do you guys put in for you max zs???

The max is in the book, page after the values for TN-S & TN-C-S. All circuits 1667ohms.

Sorry, should have added max 1667ohms to ensure operation of 30mA RCD with a touch voltage of less than 50V.

 

[ajelec10]

I am sure that a TT system does not require a time dely RCD, the maximium ZS will depend on the rating of the RCD protecting the installation/circuit in the regs iy tells you how to calculate this say you were using a 100mA rcd as the main swicth the maxium permisible ZS for any circuit protected by the rcd would be 500 ohms simply VIR. you use 50v as that is the maxium touch voltage allowed divided by the rating of the RCD so 50/0.1 equals 500ohms, remember with the 17th edition you can not protect all of the circuits with one RCD and most circuits in a domestic property will require protecting with a 30ma RCD, personaly i would install a stright board with a 100a main switch and use 30ma rcbos for all circuits.
You also can install multplue earth electrodes to lower the resistance istall them the driven distance appart.
Hope this helps
AJ

 

[KNIPEX]

thanks for the replies keep them coming guys.

so the readings are high but still withing the limit,

the rcd is 30ma, as main switch so its covering all circuits.

and more advise on this time delay rcd, and a good reason why please over a normal rcd.

it was a periodic on a house about 3 years old so 17th edtion is not really applicable

 

[not clever]

Max Zs on a TT system 1667 ohms
Disconnection time of .2
Max Ze (or Ra) 21 -200 ohm, ideal would be below 100 ohm

No need for a S type rcd now as the 30mA will suffice & complies (apart from the loss of all circuits when it trips).
The s types were/are used on TT systems to give discrimination between the 2 rcd's, but now all circuits require 30mA rcd so makes the 100mA s type redundant (having said that I have this week used 2 S types on two submains)

it was a periodic on a house about 3 years old so 17th edtion is not really applicable

But 17th is applicable as you are now testing the installation, no matter how old it is to current regulations!

 

[skirby]

Yes. It's all in the regs book.
BS 7671 Table 41.5 shows Max Zs to ensure RCD operation (for non delayed RCDs) =1667 ohms.


Note 2 below this table states that: The resistance of the earth electrode should be as low as practicable.
A value exceeding 200ohms may not be stable.

[FONT=Verdana, sans-serif]BS7671:2008[/FONT]
[FONT=Verdana, sans-serif]The note below Table 41.1 states that:[/FONT]
[FONT=Verdana, sans-serif]TT disconnection time of 0.2 secs up to 32A & 1 sec over 32A (411.3.2.4) is achieved by:[/FONT]

1. [FONT=Verdana, sans-serif]an Overcurrent Protective Device and [/FONT]
2. [FONT=Verdana, sans-serif]Equipotential Bonding (connected to all extraneous metallic parts). [/FONT]

[FONT=Verdana, sans-serif]Then the disconnection times for TN systems can be used ie. 0.4 seconds up to 32A & 5 secs over 32A (411.3.2.3)[/FONT]


[FONT=Verdana, sans-serif]On-Site Guide Section 3[/FONT]
[FONT=Verdana, sans-serif]Disconnection Times for TT circuits.[/FONT]
[FONT=Verdana, sans-serif]The required disconnection times for TT systems can (except in the most exceptional circumstances) only be achieved by protecting every circuit with an RCD.[/FONT]

Interesting stuff ? Phew!

Maybe installing more electrodes will help bring down Ze and Zs.

Under 17th Regs I would recommend installing RCBOs for individual circuits. This would give RCD protection & DP isolation to each circuit.
Obviously the customer cannot be forced to comply with current regulations but the inspector's responsibility is to make recommendations as he sees fit.