ZS on a TT system - Help!

[andyww]

Hi All

Question:
on TNS/TNCS the max Zs on a 32 Amp type B MCB is 1.15 Ohms.
On a TT system it's 1667 Ohms (17th Edition page 50 table 41.5) (and earth electrode below 200Ohms)

Why is the Zs allowed to be so high on TT??

 

[mechelec]

this is because earth faults will be cleared by the RCD main switch that protects all the circuits in a TT installation.

 

[electricAl]

Basically because

1667 ohms = 50v / 0.03A

Where 50v is considered the `safe touch voltage` (could alternatively be 25v for more demanding requirements)

& 0.030A (30mA) is the I^n (the tripping current) of the RCD protecting the circuit.



So above equation would permit a Zs upto max of 1667 ohms & still achieve disconnection via RCD before voltage rose above considered dangerous level of 50 volts

As suggested, any & all line to earth faults in a TT will be cleared by residual current device as the earth fault path has too high an impedance (typically) to achieve required disconnection times via fuses/MCBs (or disconnection atall - check out your PEFC )
So, for TT, MCBs provide overcurrent protection only.

In TN systems we do interest ourselves in achieving low enough Zs for the breaker to trip on line to earth faults, as this is considered our primary `Fault Protection` & the backup of an RCD is considered `Additional Protection` - even tho any functioning 30mA would always trip before the MCB.

So when a low impedance suppliers earth is provided, fuses/MCBs offer both overcurrent & earth fault protection...

whereas with TT, the fuses/MCBs give overcurrent whilst the RCD takes over earth fault protection

Hope thats clear enough for you

 

[andyww]

Thanks Electric Al

Good explanation - and now understand

Cheers
Andy

 

[sixfingers]

Sorry to jump this thread! So do you put max Zs as 1667 ohms on electrical cert instead of 1.15 ohms for above circuit on a TT system?

 

[electricAl]

So do you put max Zs as 1667 ohms on electrical cert instead of 1.15 ohms for above circuit on a TT system?

Yup

 

[sixfingers]

Wanted to double check on this one as spark i spoke to with about 20 years more than me in game and a lot of testing and cert filling under his belt said no to this one!

 

[electricAl]

So what do ya think he`d say if you told him that this is also the case with TN systems with 30mA RCD protection?

Cos it is

Personally i put in BS7671 derived figures for TNs, but its equally acceptable to use the 1667 figure (assuming safe touch voltage of 50)

Perhaps he should rely less on what was relevant 20 odd yrs ago & try to understand how disconnection times can be achieved today?

p.s. just noticed you`re registered with NIC - don`t just take my word for it, suggest you give em a ring & see what they say (will be quite interested myself as i`m with Elecsa & already know their view is as stated above)

Perhaps your mate might believe the NIC ?

Al

 

[sixfingers]

p.s. just noticed you`re registered with NIC - don`t just take my word for it, suggest you give em a ring & see what they say (will be quite interested myself as i`m with Elecsa & already know their view is as stated above)



Al

Yeah will do, As long as i dont get the nic tech guy that sounds like he is sitting on the loo while hes taking your call! And that did happen once

 

[jbec-john]

I had my NIC EIC annual visit last friday and asked him this exact question just to make sure I was doing it right. You do indeed put 1667 in that box according to him. That's good enough for me

EDIT:
He also said you should put the maximum Zs you find in the IEE wiring regs in for all other cases, rather than the one's found on the NIC EIC test sheet covers. A B32 for instance is 1.44 in there (If my memory serves me correctly) rather than the 1.15 found on the test sheet covers.
I think I need to get a second opinion on that one though

 

[electricAl]

I had my NIC EIC annual visit last friday and asked him this exact question just to make sure I was doing it right. You do indeed put 1667 in that box according to him. That's good enough for me

For the clarity of others reading this thread, you`re specifically talking TT here ?

He also said you should put the maximum Zs you find in the IEE wiring regs in for all other cases, rather than the one's found on the NIC EIC test sheet covers.

As i said before, use BS7671 derived max Zs values - don`t insert the 80% adjusted of the On-Site etc

A B32 for instance is 1.44 in there (If my memory serves me correctly) rather than the 1.15 found on the test sheet covers.

Nothing wrong with your memory

32A x 5 = 160A Ia (fault current required to trip MCB within 0.4 sec - for typeB would be x5 of In)

then

230v / 160A = 1.44 ohms ( Uo / Ia < or = Zs)

I think I need to get a second opinion on that one though

You have one

or should that be two?

 

[jbec-john]

Well, the job in question on my test sheet was indeed connected to a TT. But I see no reason why that shouldn't be the case for ALL circuits protected by an RCD.
That said, any circuit on a TN-S or TN-C-S with a high enough reading to go over those found in BS-7671 would indicate either a very high Ze or an R1+ R2 that needed a good looking at. I'm sure it's possible, but highly unlikely.
Have you ever come across a 'sound' ring main with a Zs higher than 1.44?

Edit
Oh yeah, and cheers for the second opinion. My mind is settled

 

[Des 56]

Because earth fault protection is provided by an rcd the figure for maximum zs should be 1667 ohms
The figures for TN systems are not noted for TT
IT is worth recording the actual ZS of a TT system for future reference when a PIR is carried out
The rcd in TN systems is for additional protection
The max zs of the primary protectrion in a TN system by the MCB should then be noted

 

[andyww]

Thanks to all for the discussion and clarity on this question - by the way - I too am NIC registered and I did call the tech helpline and was told that the answer is........... 1667!

Cheers