required Ze

[Frank Faraday]

Why is the permitted Ze higher for a TT system than PME?

Is it because TTs are required to have RCD protection?

If so, if an RCD is present in a PME system, can we ignore the figures in the OSG for PME and refer to the higher TT Ze permitted figure?

 

[Evans Electric]

No you can't ignore them , each system is different ,  those are the recommended figures for those systems .       Difficult to get a  TT system  Ze  down to the PME or TN-S  level  so Voltage operated trips were used  before we  became knee deep in RCDs .          They were somewhat hit & miss I believe , now totally banned. 

Even with every sub circuit RCD'd  @ 30mA   you still have the upstream 100mA  on TT   .  

And as is often said ,  the system relies upon  the  RCD  being regularly tested & remaining in working order.     Yeah like that happens.

 

[Lurch]

Why is the permitted Ze higher for a TT system than PME?

Is it because TTs are required to have RCD protection?

I think you've got that a bit jumbled up. TT's have an RCD because Ze is high.

If so, if an RCD is present in a PME system, can we ignore the figures in the OSG for PME and refer to the higher TT Ze permitted figure?

You could do that, technically it's not wrong but it's a bit of a half arsed way of doing it. If you have to do that then you are just masking a problem. The Ze on TT is higher, so we allow for that. The Ze on PME is lower, so we design around hat. If the Ze on PME is high it means something is wrong, so it need fixing.

 

[Frank Faraday]

So sometime in the past, someone decided what would be an acceptable minimum Ze for safety. Surely this figure should be regardless of the type of earthing system used?

 

[Andy™]

different earthing systems have different characteristics therefore they each have their own requirements

 

[Evans Electric]

Frank,    in my experience you'd have a job to get an earth rod down to  0.35 ohm    or 0.8  

We did an inspection last month ,    no incoming earth,  an earth rod  so TT,    no RCDs   ,  Ze  was over 700ohms  .   So what I'm saying is  a TT system  has to be approached  differently to the others .    The safety of the install was relying on the bonding  to gas & water really ........until it is changed to plastic.  

 

[Frank Faraday]

different earthing systems have different characteristics therefore they each have their own requirements

What different characteristics?

 

[Frank Faraday]

Frank,    in my experience you'd have a job to get an earth rod down to  0.35 ohm    or 0.8  

We did an inspection last month ,    no incoming earth,  an earth rod  so TT,    no RCDs   ,  Ze  was over 700ohms  .   So what I'm saying is  a TT system  has to be approached  differently to the others .    The safety of the install was relying on the bonding  to gas & water really ........until it is changed to plastic.  

I'd just like to know why there's such a huge disparity between 200 ohms (or whatever it is) and 0.35 ohms. It seems leniency is given to TT just because it can't get as good a result as a PME. In reality, isn't it the case that all you really need is a sufficient path to earth to be able to trip the RCD ? (obviously where RCDs are present in the installation).

 

[Andy™]

i take it you're no relation to Michael then

 

[Evans Electric]

200 ohms is given as  the highest reading before it starts to get "unstable"        They can't give set figures  because of the nature of the system .       You may get 20 ohms in  a wet winter  but what will it be in the summer. ?  

 

[Lurch]

So sometime in the past, someone decided what would be an acceptable minimum Ze for safety. Surely this figure should be regardless of the type of earthing system used?

Again, you have got this backwards. 1667Ω is not "what someone decided in the past". Have a look at where that figure comes from.

 

[Frank Faraday]

Again, you have got this backwards. 1667Ω is not "what someone decided in the past". Have a look at where that figure comes from.

I'd be interested to know - can you point me in the right direction?

In the meantime, can you tell me why, for example, a Ze of say 20 ohms is acceptable for a TT but not for a PME

 

[Andy™]

I'd be interested to know - can you point me in the right direction?

7671

 

[Sidewinder]

Frank,

Welcome to the forum.

You are viewing this in the wrong way.

Ze is not an allowable system parameter, really.

Let's start with TN-C-S, do you know where the 0.35 value is derived from?

Think about the science and engineering behind the supply presentation, and see what you come up with.

 

[steptoe]

@Frank Faraday

it may be useful if you tell us your background, knowledge of electrical systems etc,

then we can better base our replies to how much you actually know, and understand, or dont, whichever may be the case

 

[binky]

Try a different view point, for a house on a TT system I would expect the cct resistances up to the rod to be the same as any other earthing system - if they aren't you have a faulty cct, loose screw in a socket of something like that. So your dead testing R1 + R2 should basically be the same as any other property. If you had 1666 Ohms on the R1 + R2 and a rod at 1 Ohm, I would expect a fire under the floorboards! Even if that apparently complies with regs.

That's the way I look at it.