earth sparking when connecting to metal frame of building..HELP!

[dyland22]

Hi,installed a 3 phase board in a commercial premises...took the earth bond off the metal frame work of the building to test its continuity,all other tests give very good readings,the only test that could be looked at as being suspisious is the pscc and efli values, because L1 is giving a very different reading to L2 and L3 which are almost identical...reconnected the earth bond and it sparks as i touch the frame with the cable!!!..turned off the main isolator feeding the building to confirm it wasnt anything i did,still sparks!!!!....never seen this before,please help?..cant pick up any voltage from the frame work to neutral by the way!!

 

[WDMDL]

TN-C-S supply? Parallel path? You could get a high current flowing with less than a volt of potential difference.

Have you got a clamp meter to check current flow through the bonding conductor?

Another thought... As your L1 readings are quite different from L2 and L3, there could be a severely unbalanced load (elsewhere on the supply!) causing a higher than expected neutral current

Simon.

 

[Darkwood]

Firstly give your readings, secondly if the bond is sparking when you attach to structural steel then you have a P.D. between your MET and the steelwork of the building, this can arise for a few reasons and may be anything from a fault within the installation and bad earthing or an external fault from a seperate installation using your steelwork as a route ack to earth, there may even be a DNO issue here with the supply to local premises.
I assume main earth bond and all circuit earths are connected when you get this sparking?

 

[Darkwood]

Firstly give your readings, secondly if the bond is sparking when you attach to structural steel then you have a P.D. between your MET and the steelwork of the building, this can arise for a few reasons and may be anything from a fault within the installation and bad earthing or an external fault from a seperate installation using your steelwork as a route ack to earth, there may even be a DNO issue here with the supply to local premises.
I assume main earth bond and all circuit earths are connected when you get this sparking?

I also assume any funtional earth circuits are giving good results but a small spark may arise if the steel work provide a better return path.

 

[Knobhead]

What are the earthing arrangements at the service head?
As WDMDL said get a clamp meter on the earth, with large metal framed building it’s not unusual to get a bit of current flow. Is this the only point that the metal work is bonded?

 

[Darkwood]

Just to add to this thread the reason a TNCS system has a requirement for larger earthing(respective of supply size) is a eddie current system can be created circulating around the earth bonding from your installation and local installations and this is normal thus accounted for in the reg's for larger bonding requirements in TNCS.

 

[dyland22]

thanks for the quick reply...the results were as follows

TNC-s supply
MEM isolator fed with 35mm tails dropping to 25mm through 80amp fuses to main 3phase board
16mm earthing conductor from main board back to distributor supply neutral.

ze/L1-0.14
ze/L2-0.10
ze/L3-0.10

PEFC/L1-1.68
PEFC/L2-2.2
PEFC/L3-2.2

PSCC/L1-1.51
PSCC/L2-2.2
PSCC/L3-2.3

The sparking was still there when the main isolator was off!!
All other test are well within range,dont have a clamp meter,never needed one to be honest!
The building is split into two units so maybe there could be a fault next door..
How dangerous do you think this is or is it something common?

 

[dyland22]

sorry forgot to say yes all other cpc's are connected during the sparking..

 

[dyland22]

Gonna add something else which maybe will help... a long shot but worth a go!!

I have been talking with the supply company of the lights I have installed within the building in question,we have installed 13 x dual flouresent lights in the laboratories..standard 5ft 58w starter and ballast with daylight tubes...all the lights have been a nightmare,not starting properly,and flickering like mad on the ends of the tubes,also very noisy,do you think this problem could be affecting the lights?

 

[Darkwood]

Are the fittings electronic ballasts or do they have starters, also check for volts drop at fittings and double check circuit tests regarding said lighting there could be a broken or high resistance neutral and maybe they have found alternative return path through the earthing system. Double check insulation resistance on lighting circuit especially between N / E, other areas to look at is loose neutral or Live conductor or incorrectly clamped wire. Take an effected fitting and wire up to 13amp plug and test in socket if it fine you know you probably have a wiring issue.

 

[ackbarthestar]

Are the fittings electronic ballasts or do they have starters, also check for volts drop at fittings and double check circuit tests regarding said lighting there could be a broken or high resistance neutral and maybe they have found alternative return path through the earthing system. Double check insulation resistance on lighting circuit especially between N / E, other areas to look at is loose neutral or Live conductor or incorrectly clamped wire. Take an effected fitting and wire up to 13amp plug and test in socket if it fine you know you probably have a wiring issue.

This maybe true and may be another fault sitting there, but it doesn't explain the sparking between the MPB and structural steelwork when the Installation has been isolated.

This is either a fault in the next unit or an accumulation of Network neutral currents using the paralleled earthing as a return path and IMO is perfectly satisfactory for the installation type and earthing system.

I would make sure that the MPB is as table 54.8 for the size of supply neutral

 

[Darkwood]

This maybe true and may be another fault sitting there, but it doesn't explain the sparking between the MPB and structural steelwork when the Installation has been isolated.

This is either a fault in the next unit or an accumulation of Network neutral currents using the paralleled earthing as a return path and IMO is perfectly satisfactory for the installation type and earthing system.

I would make sure that the MPB is as table 54.8 for the size of supply neutral

My previous posts explain TNCS and circulating eddie currents in the combined Earth/Neutral network which isnt too uncommon, hence larger bonding conductors required, i was just explaining his lighting issue in last post which is probably a seperate problem.

 

[dyland22]

cheers darkwood..very good advice,i will double check and hopefully this does have smething to do with the sparking bond connection..the lights were installed by someone else who is usually pretty good,think i will take them all down and check all connections,including manufacture connections..

If you need something done properly do it youreslf!..every time

 

[Darkwood]

go to the first fitting or connection where the problem is and usually you'll find your problem there as all the other fittings will prob' looped off a bad joint in the first fitting, if it doing it over different rows switched seperately then look where the first neutral is connected or look in the switch at live loops. This will save taking them all down randomly and usually gets quicker results, good luck !

 

[WDMDL]

all the lights have been a nightmare,not starting properly,and flickering like mad on the ends of the tubes,also very noisy,do you think this problem could be affecting the lights?

Doubt it... it's a separate issue.

Are they all on the same circuit, or split between circuits / phases?

Noise from the ballasts is generally down to one thing.... a poor quality luminaire. Usually the method of mounting the ballast plays a big part, though it may be the ballasts themselves.
It may be you've used this brand in the past and they've been fine.

External causes of ballast noise are due to a poor supply waveform... possibly an arcing connection on the circuit, or some other load corrupting the mains supply.

Poor starting of a 58W fitting... you've only 4 components to think about... the lamp, starter, ballast and PFC capacitor.
However if you haven't used PFC corrected fittings, the voltage spike applied to the tube to strike it by breaking the inductive circuit may be reduced if the cable run is long.

As all the ballasts, tubes and starters will be of the same brand, I'd try changing one lamp in a couple of fittings to another brand.
Allow a few hours for the lamps to burn in, then you could compare the running voltage of each lamp by measuring at the starter socket.

I no longer have my Philips lighting data book which gave standard voltages and currents for each lamp type.
If the daylight lamps are running at a higher voltage, this may be too near the conduction threshold for the starters.
A cheat would be to try using the starter type for 6 and 8-foot lamps.

Flickering at the ends of the lamps may be down to a low lamp running current, or possibly that the lamps haven't stabilised yet.

Try another brand of luminaire at the final point on the circuit - that will have the least favourable conditions. If it starts fine....

Simon.