TN-S and RCD's

[Graeme Harrold]

Just done a load of inspections on a caravan park where the entire distribution system is TN-S. Problem is that quite a few of the Ze's are above 0.8 ohms stipulated as maximum. The question is, does the use of RCD's mitigate the above parameter readings? or will they have to get the Ze's down.......

 

[pevvers]

Just done a load of inspections on a caravan park where the entire distribution system is TN-S. Problem is that quite a few of the Ze's are above 0.8 ohms stipulated as maximum. The question is, does the use of RCD's mitigate the above parameter readings? or will they have to get the Ze's down.......

Are we talking about your Ze's being at the point of a DNO fuse, or are they at the end of submains? The 0.8 figure is only used when talking about the main supply to an installation from the DNO, nothing after this.

To the original question, yes, an RCD will mitigate this problem. But if it's that high then I would be looking at volt drop problems as well if I were you, as it sounds like a bit of a dodgy install, which is to be expected really on caravan sites.

 

[Graeme Harrold]

Thanks for the reply,

Highest Ze I got was 79.7 ohms, but the majority are in the 1 to 1.8 ohm range. Thankfully they have back to back RCD's just to keep things safe (but not conveniant for the guy who comes out to reset them!!!) Will make recommedations to get the Ze figures down as some of the dist panels are in a poor state.

 

[Deleted member 9648]

79.7 ohms is way high for a tns.....are you sure the earth has not been separated and spiked?

 

[pevvers]

79.7 ohms is way high for a tns.....are you sure the earth has not been separated and spiked?

Yup, completely agree. The relatively low ones could be extremely long runs from a TN system, but the 79 ohms is either a TT or the earth has been broken somewhere...

 

[Graeme Harrold]

79.7 ohms is way high for a tns.....are you sure the earth has not been separated and spiked?[/Q

Given the state some of the SWA is in, the armour is quite badly corroded in places and this is probably what is wrong here. Fortunately I am only doing the reports, someone else can find and sort it as they have on-site sparks........

 

[pevvers]

Given the state some of the SWA is in, the armour is quite badly corroded in places and this is probably what is wrong here. Fortunately I am only doing the reports, someone else can find and sort it as they have on-site sparks........

That's what I thought the problem to cause Ze's of that magnitude would be. I hope I never have the misfortune of staying at that park as the on site sparks don't sound like they're up to much!!