Origin of Ze Values...

[Silly Sausage]

Where did the nominal/limit values of Ze for TNS and TNC/PME originate from?
I mean the values that are listed in OSG and various other publications, but why?

Does it really matter? As long as your Zs for a cct falls within the max for the protective device...

 

[Darkwood]

In short circuit a cable has to cope with currents of several times + its rating but the duration of this current is vital to the damage that may occur... if too long then the insulation and environmental protection could have its characteristics compromised .. its these well researched limits that define Zs values.... nothing to do with personal safety just protection of the installation.

Tnc/pme can be subject to circulating currents in the earthing system under fault conditions and can means an installation external to the one your on could be larger and if the supply has a fault it could use your earthing system as a route... so the earthing system has to allow for this.

 

[Des 56]

The question was the origin of Ze values

I would have thought it was more to do with permitting the ability of an installation to stay within the maximum Zs values after final circuits are added, the difference between Tnc-s and Tns values being more to do with the integrity of the supply system and its connections

The value of a Tns seperate conductor.a lead or steel sheath, rather than the copper neutral of a Tnc-s making it more vulnerable to higher resistance than what would be expected in a Tnc-s conductor

 

[jimmymac]

I'm surprised E54 hasn't been on this thread yet