Parallel Earth Paths In Steel Re-Inforced Concrete

[binky]

we have just converted a carpark to LED lighting which involved removing all the old metal light fittings and installing plastic IP units. We tested circuits before commencing work, and found afew small issues. Today we retested circuits and found R1 + R2 values have increased. Double checked lots of our connections but can't find anything wrong with those. We haven't changed the actual circuits or cable runs which are run in steel conduit and steel trunking with some form of shared earthing system - there's about 6 actual earth cables for 16 circuits, and large parts of the car-park are conduit earthed. Car park is 1970s build and on original wiring. After a good days head scratching and re-testing we are at a loss to explain increase in R1 + R2 values, though live testing for Zs seems to give significantly better results. So we suspect that metal fittings screwed to concrete via large metal screws has possibly been giving parallel earth paths via the steel beams and re-inforcing steel works. Question 1/  has anyone else ever encountered such a thing before, or am I barking up wrong tree?

Question2/ would parallel earths via structural steel be considered acceptable if delivering acceptable Zs values?

my opinion to Q2 is 'not really' even if the steel isn't going anywhere soon!

 

[binky]

varies, but most circuits proir to tinckering around 1.1 ohm, now increased by around 0,5 ohms kind of across the board, though 1 circuit has shot up to 70 ohms R1+R2 or 4,5 Ohm Zs

thing ig we can't think of anything we have done to cuase an increase, on 2 circuits previously out of spec, values have actually dropped slightly.

 

[Andy™]

well structural steel can be used as an earth electrode

 

[binky]

never seen it used when encased in concrete though Andy - still shouldn't rust LOL

incidentally I applied 240V to a florrie tube today (forgot I had converted for LED lamp) - you're right, they don't like it, went bang a treat!

 

[Andy™]

incidentally I applied 240V to a florrie tube today (forgot I had converted for LED lamp) - you're right, they don't like it, went bang a treat!

how could you forget? or was the LED tube already removed? all the more reason not to alter the internal wiring

 

[Marvo]

Check out Ufer earthing.

 

[binky]

how could you forget? or was the LED tube already removed? all the more reason not to alter the internal wiring

been lying around for a while - spare fitting from a previous LED job, The led tubes have built in drivers so we apply 240v at the contacts

Check out Ufer earthing.

nice one Marvo, can't say I'ver ever heard of it before. Think the issue has been 'accidental' Ufer earthing. We hit lots of steel work whilst drilling holes for the new fittings, and think many of the old units had done the same, but being metal formed lots of parallel earthing paths.

 

[binky]

this is the actual car-park with LED installed - vastly superior lighting. Give you some idea of the construction. you can see some of the metal conduit near front of picture

 

[Marvo]

Lol, it's a system we regularly rely on but I doubt there would be much need for it in the UK  with regular rainfall and real soil instead of pure sand. The steel conduit that's mounted surface on the concrete won't provide much of an Ufer type earth. Ufer earthing works best when the concrete slab is on the ground and the metalwork is completely encased in the concrete. That said the reinforcing rods will connect the upper slab to the lower slabs via the steelwork in the pillars so if the steel conduit is in contact with the re-bar at some point it could provide a decent parallel earthing path..... problem is the path might not be reliable over time if the contact is unintentional. 

 

[sprocketflup]

Check out Ufer earthing.

Every days a school day

 

[binky]

changing tack slightly, I now need to patch up poor earth paths no doubt where conduit/trunking has poor connections. I was thinking of by passing said areas with earth straps. Most circuits are on 20Amp breakers, so was going to size said straps for 20 Amps, working on basis that only 1 circuit is likely to be faulty at a time. Am I correct in taking that approach given that certain parts of the metal trunking is serving as earthing for 16 circuits?

 

[Steve3948]

Binky, I thought all your test results were satisfactory though, is there a requirement to do more, or have you made to much money on the job

 

[binky]

the test results were ok for most of the circuits when we tested prior to starting the job, but on re-testing we have found several circuits have suddenly jumped by anything upto 70 Ohms, and are a bit of a loss as to explain why. Now it could have been our handiwork, but we have gone through several circuits with a fine tooth comb, and get these new figures even at the the very first light fitting on the circuit. So in short the circuits can be shown to now fail on parts that we have not altered at all. We suspect the old ffittings being all metal construction were making contact with the steel work of the building and providing parallel earth paths, where as the new fittings being typical plastic construction are not doing this, revealing that the circuits that we originally though to be ok, were actually duff all along.

 

[batty]

The threads in the conduit have rusted so is no longer serviceable as the protective system. I think you are going to have to run a separate CPC in the conduits if possible.