Earth Loop Problem

[Jim2287]

if your earth loop is too high for the device, how can you fix the problem ?

upgrade Earth conductor CSA
upgrade live conductor CSA
or, change the device for one which trips easier

any other ways ?

We got a problem after a test was carried out on a school...
A fuseboard is supplied from the main intake in a school, supplied in MI cable, the impedance was measured at 0.41ohms which was too high for the fuse protecting it to break quick enough

-As it was MI we checked the lock rings etc, which were loose, tried it again and no change in value
-Tried putting a banjo on, no change
-Tried running an additional 10mm earth - no change


the Ze is ok, and so is the EFLI at other boards around the school, just not this one

Any ideas ?

 

[Steve]

Fit a 100mA RCD

 

[Jim2287]

hmm do you think that would be ok as a resolution ? that only breaks if there is a fault which leaks to earth, not overcurrent

perhaps we can get enough on the mims to put a small enclosure with new breaker "in-line" ... would that be acceptable ?


eg

100A Fuse -------100AMP Breaker----------------Fuse Board
Doesnt break......Does break



The switch gear now is 3 phase, only one of the phases has the problem so if we cant do inline we would have to change the whole lot

 

[RF Lighting]

Your breaker will still take care of overcurrent regardless of the loop value. If the fault was to earth the bad earth path may not cause the OCPD to operate but the RCD will take care of this. I don't really think that a 100mA RCD is the right thing to protect a submain in a commercial installation such as a school. (Crafty does have a sense of humor).

How long is the run of MI cable? What size is the cable (CSA and No. of cores)? What is the OCPD feeding the cable (size, type and BS No. if poss)? What is the Ze for the installation? and any other information that might be relevant.

 

[securespark]

Yeah, need breaker type and size for proper calculation.

Adding an RCD will as Crafty pointed out, allow a much higher EFLI reading, however, 0.41 sounds a good reading, but it all hinges on what breaker is protecting the circuit, and whether the circuit requires 0.4s or 5s disconnection time.

 

[Jim2287]

How long is the run of MI cable? What size is the cable (CSA and No. of cores)? What is the OCPD feeding the cable (size, type and BS No. if poss)? What is the Ze for the installation? and any other information that might be relevant.

cant remember off the top of my head now

the fuse was a BS88 100amp

the run of cable was about 30 meters at a guess, but other fuse boards were further away and didnt have problems

the MI was 25mm but thats a real guess since i dont really see much MI to get to recognise it, about 7mm diameter

Ze i dont know but it was legal and didnt cause problems for other boards


pretty useless info with all guesses



what is really strange to me is the fact that adding a 10mm additional earth didnt change the reading even slightly ?

 

[supersparky]

should you really be doing this in a school if you have to ask on a diy forum?

anyway, The earth of a 25mm MI cable will be large even if its just the sheath, are you sure theres no break in the sheath of the cable at some point?
is there an extra earth core?

RCD isnt ideal but can be used or an RCBO

if the board requires 0.4sec then what about an RCD incomer to the board? or RCBOs for 0.4sec circuits?

or can you replace with SWA of say 35mm? prehaps incorperating an extra core?

whats the Ze at intake? can you convert to PME?

are you sure that this is the only intake and the rest of the school isnt off another intake with lower values?

 

[Jim2287]

well im not qualified but the people i work with are... i just thought id see if i could learn something from it, or even advise something they hadnt thought of


replacing the cable i think would be last resort

there are no breaks that are visable

the boards in the school are old MEM rewireables 3036 so putting RCD's and RCBO in them might be difficult, but external to the board could be done


BS 60898 breaker
Zs = 240v / 5x 100amp = 0.48ohms

if we put a 60898 inline then 0.41 ohms would be acceptable ?
want to avoid RCD's if possible because the wiring in the school is quite old and dont want nuisance trips

 

[supersparky]

well if its old and in such a state that it would trip an RCD then that a fault, isnt it and so the instalation is unsatisfactory........

RCD only does its job and if the wiring is in such a state.....

but agree, RCDs in schools are a bad idea if possible to avoid due to teachers not having keys to DBs and lack of trained "RCD resetting personel"

also, 3036 fuses....lots of schools are full of em but that would be a good idea to replace.... if possible

 

[RF Lighting]

You say you tried pulling in an additional 10mm² earth cable which had not effect. Is this cable still in place? if so can you connect it to the sheath of the MI with something like an earth clamp, making sure the MI sheath is polished clean with steel wool, then take a low range continuity tester to the other end of the cables and test between the 10mm² earth wire and the MI sheath (on another well cleaned bit of sheath). This will give you true confirmation of wether there is a break in the sheath (which is unlikely because even a small knick in the sheath would cause the magnesium oxide in the cable to absorb moisture and the insulation resistance values would drop until there was a dead short in the cable thus operating the OCPD for the cable) You can also get a rough value for the resistance of the MI sheath by deducting the value of the 10mm² earth wire from the continuity reading (either by testing the earth cable end to end or by working out the resistance from the milliohms per meter). 10mm² has a resistance of 1.83 milliohms per meter multiplied by the 30 meters you guessed = 0.0549 ohms

Sorry its a bit long winded and I hope this makes sense.

 

[Spark123]

BS 60898 breaker
Zs = 240v / 5x 100amp = 0.48ohms

if we put a 60898 inline then 0.41 ohms would be acceptable ?

This depends on the conductor maximum temperature of the cable (for MICC it is higher than 70deg C), 0.48ohms being the maximum efli at the maximum conductor temperature of the cable, if the efli measurement is taken at a lower cable temperature the maximum measured value needs to be adjusted accordingly.
What is the code on the pots on the end of the MICC? Should be something like 2L25 or 2H25.

 

[supersparky]

RF is spot on there

i was thinking more along the lines of a joint rather than a break as you say magnesium oxide sucks more than a back street whore so you would notice

prehaps a numpty has used a plastic joint box and not linked...

 

[RF Lighting]

magnesium oxide sucks more than a back street **** so you would notice

 

[Jim2287]

would taking R1+R2 be able to indicate a break in the earth ?

they did not test that ... or did not write down a measurement for it

if there is a break, the loop still works but not as well ?


Rf the stuff u said makes sense yep

we just ran the 10mm accross the floor before wasting our time for no benefit, so we can run it in again if R1R2 wouldnt show it

 

[supersparky]

r1+ r2 wouldent no

Unless you can isolate the MI sheath at one end

other wise it will likly take the alternate route that the EFLI test took

you should really be able to calculate what your looking for

but we need to hear the ZE!!!!!

and a rough guess at cable length would do

plus how many cores

from this you can get a figure on resistance * length of run

if its way off the mark then theres a problem

running a 10mm earth should make a difference though as its a conductor in paralell however id probrably run a 16/25 if the MI is 25

arnt these guys interested in remedials!??!?!!

if this place is within herts/ london i could hazard a guess at a company that gets stuck on this kind of simple problem

however, you say EFLI at the other board is ok, is this because their runs are shorter? or their protective device is lower?

prehaps this board has the highest fuse?

prehaps the Ze is 0.37 which if its 30 meters of 25mm 4 core (assumeing 3 phase) would tally about right at a stab in the dark guess

need lots more info from you