high zs on caravan site

[DAVID BUSHELL]

having carried out a periodic inspection on caravan site the submain cables were protected by 60 amp mccb . the maximum zs permmited for this type is .48ohms we are getting any where between .78 and 4.89 , the high readings are due to the lenght of runs and bad connections , am i right in that we can put 100/300ma rcd inline with mccb

 

[Adam_151]

As far as I understand it, you can use an RCD as a last resort if you can't comply with the tabulated values of Zs, however

1) It'd have to be time delayed to offer discrimination against 30ma devices on pitch supplies, and socket outlets etc

2) If its due to a bad connection, then it needs sorting out as its only going to carry on degrading and one day you are going to be left with no earth at all, in this case, fitting an RCD is akin to repainting a wall that is mouldy due to a leaking pipe, it may fix the problem short term, but theres only one way to deal eith it properly...

 

[ColJack]

drive an earth rod into the ground.

bad connections ought to be fixed..

 

[DAVID BUSHELL]

all bad connections have been rectified with new glands and terminations etc, its just that ther are really long runs and possible underground joints

 

[Lectrician]

drive an earth rod into the ground.

bad connections ought to be fixed..

What goods that going to do?!?

 

[mst]

na s type RCD is the best option then if your sure its down to the lenth.

 

[ColJack]

parallel earth paths..

same as they do for PME..

or if protecting with an additional RCD, the second caravan pitch diagram in your very own link...

is it not allowed to install an earth rod if you have a TN system?

where are you getting your Zs maximums from anyway?

i get 0.76 for a B type 63A breaker..

 

[shortcircuit48]

Coljack

Can you please explain

"is it not allowed to install an earth rod if you have a TN system?"

 

[pop123]

having carried out a periodic inspection on caravan site the submain cables were protected by 60 amp mccb . the maximum zs permmited for this type is .48ohms we are getting any where between .78 and 4.89 , the high readings are due to the lenght of runs and bad connections , am i right in that we can put 100/300ma rcd inline with mccb

the maximum zs permmited for this type is .48ohms we are


Calculated or measured applying rule of thumb?

 

[Lectrician]

There is nothing wrong with putting a ROD down on a TN system, but it is a pointless exercise. If the impedance is high due to corroded glands etc, these need fixing. If the impedance is high due to length of run, a rod is not going to help!

Coljack - What link of mine? Are you referring to rodding down caravan hookups on a PME system? In this case both earthing systems MUST be kept seperate. See my drawing if you have found a link.

 

[nozspark]

Please be aware that whilst you may have replace all the glands, that your high impedance might be due to your cable run being too long for the size of cable being used.
A couple of years ago I did a job like this and found that they had 100+ meter runs of 6mm 3 core SWA fed off a C50 MCB feeding 4 pitches (each mobile home pitch had it's own 30mA RCD and 10A MCB) - any fault whether L-E or L-N at the end of the cable would not have been sufficient to trip the MCB!!

 

[Spark123]

having carried out a periodic inspection on caravan site the submain cables were protected by 60 amp mccb . the maximum zs permmited for this type is .48ohms we are getting any where between .78 and 4.89 , the high readings are due to the lenght of runs and bad connections , am i right in that we can put 100/300ma rcd inline with mccb

What type of earthing arrangement?
With long lengths of cables volt drop will also be an issue. What size & type of cable is the submains? Any bad connections will need to be found and sorted properly, even if underground.