TT Earthing issue

[Joey d]

Hi All,

This might seem basic but I could use some advice.

In a domestic property the earthing arrangement is TT. A new shower circuit needs installing, but have done some testing the Ze is 74 ohms, Zs@db 4.8ohms, Ipf@db 49a
 

Now obviously on any new circuits to be installed, they will have a Zs value higher than the permitted value in BS7671. 
 

My question is should I contact Western Power to have TT changed over to PME? 
What needs to be done in order for a new circuit to be installed and then signed off as satisfactory in a EIC?

 

Many thanks for your help guys

 

Joey

 

[Risteard]

Your PFC most certainly won't be 49A - there would be ridiculous Volt drop otherwise. That is the PEFC. Your PFC will be the PSCC. 

Surely it's obvious how fault protection would be provided on a TT system. 

 

[Doc Hudson]

Now obviously on any new circuits to be installed, they will have a Zs value higher than the permitted value in BS7671. 



I suspect you may be trying to follow guidance for TN systems (e.g. 411.4). Whereas TT systems have separate guidance in Chapter 41 protection against electric shock. Think how the existing circuits could be meeting the requirements of BS7671, or are you suggesting that the whole installation is unsafe?

Doc H.

 

[Joey d]

Sorry I meant 490a PEFC. 463a PSCC

 

[Joey d]

I suspect you may be trying to follow guidance for TN systems (e.g. 411.4). Whereas TT systems have separate guidance in Chapter 41 protection against electric shock. Think how the existing circuits could be meeting the requirements of BS7671, or are you suggesting that the whole installation is unsafe?

Doc H.

 

[Joey d]

I only have experience with TN systems and am struggling to see how on a TT system with a higher Ze will meet the circuit breaker Zs values in table 41.3. I’m obviously missing something but don’t know what? 
 

Joey

 

[Evans Electric]

Have a read in the Regs for  the requirements for   TT  supplies .  

My question is should I contact Western Power to have TT changed over to PME? 

I found , generally ,  if its a TT   theres  a reason  in urban areas  ,  usually a problem with the network  .   We get it a lot in parts of Sutton Coldfield.   

 

[Sidney]

Is you PSSC enough to disconnect the OCPD within the required time for a TT system?

Do you have additional protection?

If the answer to both of the above is yes then you should be good to go.

PSSC will be enough for short circuit faults and a 30mA RCD will be suitable for earth faults.

For some strange reason I like working on TT systems.

 

[binky]

OK, best way to think of TT circuits is that if R1+R2 is more or less what you would expect from a TN-C-S or TN-S, then it's a good circuit.  In theory you could have a rod at 500  hms  and R1+R2 at 1000  hms  and it would still comply with the 1667  hms requirement when protected by a 30mA RCD, but personally I would be unhappy with that!