Service fuse

[plugwash]

it all depends

there may be situations in which the old TN-S has failed and they can't safely PME that TT is the best/easiest way to go for everyone involved.

however i belive they should introduce a rule that wherever feasible (including ALL runs from new stepdown transformers to new propertys) that TN-S should be used.

 

[ban-all-sheds]

TN-C-S has a lower typical Ze, which gives you more breathing space for your installation.

And more complications taking the supply to outbuildings.

 

[plugwash]

TN-C-S has stricter rules on the maximum allowed Ze

what the typical values are is harder to tell (maybe one of the sparkys here could dig through thier archives of test certs and give us some average values for Ze for different earthing systems) but i bet they give you FAR more room than using the worst allowed case figures.

the danger from a TN-C-S is not within the equipoential zone its to things connected to the TN-C-S earth but outside the equipotential zone.

 

[Lectrician]

I have NEVER seen a TN-s nearing 0.8ohm!!

There is always a good chance on a TN-s system that the Ze will be lower than the P-N loop. (measured to determine PFC). Especially with old lead cables. Not so much with the modern split con, or 4 core con.

 

[plugwash]

i hope in such conditions you record the highest PSC (ie if the PE PSC is higher than the PN PSC you record that)

 

[securespark]

Just for the record, I've asked my REC to remake earth connections onto the cables loads of times, and they have. I point out that an EC14 on the sheath is NOT good enough, and they now have a responsibility to maintain a sound earth connection. So many times I have been fobbed off by p r a t s in the office saying "Not our responsibility anymore, mate" when I know damn well it is. If you know the rules and regs, and stand your ground, then they have to keep their side of the bargain, but I admit, many would be put off.

I've seen clamps so loose, they're hanging off and some cables have a indentation where the clamp has been. So many times, the REC has said, "can't you tighten it up?". My reply has been "can't you?"

 

[plugwash]

is there a way to tighten those clamps from a distance.

if i was feeling especially evil it sounds like fun to get them on tape saying can't you tighten it and then do so hoping the cable will go bang

would be nice to have a video of just what an overtightend clamp can do to a nice thick lead and paper cable on a nice high current source.

 

[Lectrician]

i hope in such conditions you record the highest PSC (ie if the PE PSC is higher than the PN PSC you record that)

Thats why I mentioned it

The lowest loop reading (P to E or P to N or P to P) will give the highest fault current.

PFC Prospective Fault Current. PSC Prospective Short Circuit Current.

 

[kai]

Has anyone EVER experienced for real, a tenby clamp damaging an old lead cable? It's probably scare-mongering again!

 

[plugwash]

i guess it depends what you mean by damaging

i haven't heared anyone here report actually seeing one go bang but that may be because they aren't dumb enough to try and tighten a loose clamp further into the dent it leaves in the cable

sorry but as far as im concerned a cable with a deep dent in it in it IS already damaged and sooner or later if you keep tightening that clamp it is going to destory the cable.

furthermore do you really want a bad connection that could leave the system with no solid earth or get seriously hot (possiblly again doing severe damage to the cable) during a fault.

 

[kai]

Bit sarcastic here......Well, at least it will force the REC to replace their crumbling cables, if it causes repeated failure of their supply fuses, or endless tripping of their circuit breakers in the local substation - HA HA!

 

[Lectrician]

Our rec are having trouble with a dodgy batch of resin from the 60's

Resin joints going bang weekly recently!! Good fun.

Ever seen a fault backed by a fuse in the sub?? LOL. My apprentice drilled through a REC cable that had been pebble dashed over on an exterior wall - that was a good one. Also seen a guy putting up signs on telegraph poles - we was blown of his ladder when he drilled right through the pole into the cable coming down the reverse!

 

[ban-all-sheds]

I don't think a Tenby is in any way a good idea...

I did read (I think at the IEE place) a post from someone who worked for a compay that developed a collar type of clamp, so that the pressure was evenly distributed, but I don't know what became of it.

If you were going to use a clamp (e.g. in a real emergency) I think a jubilee clip would do less damage....