Site cabin EICR

Hello Paul Brewerton, 

The distance and true value of electrode resistance when perfect and according to the measurement principles, The following condition must be met: The difference between the 3 measurements (electrode of the voltage circuit P) does not exceed 10%, If not, you need to change the distances or direction.

Thanks Benyamin,


From IET GN3, Pg 64
For TN-S systems and generator supplies electrode resistance values may not have been specified, as there is a separate metallic earth conductor and electrodes often simply provide a local reference earth.


So from that I am taking that there isn't a minimum resistance reading we are looking for?

What is the rating of the generator?

Imagine that somehow wiring or a fitting in the cabin fails, and the metal door becomes live, or that someone walks out the door with an extension lead that is damaged.  If someone could get between the mud outside, and some exposed live parts,  will the ADS operate without exposing the user to a dangerous voltage for a dangerous time? 

That is the killer question.

By which we mean can we manage knock off times of 230V in half a heartbeat period, (0.2 seconds)  or 110V in a heartbeat period  (0.4 seconds), or anything less than 50V more or less for ever (sub kilo-second perhaps ?)

If you have an RCD as the first thing in the line, then we can turn that round, what resistance would operate the RCD suitably fast, with a voltage drop such that no more  than 50v reached the victim. For a 30mA RCD  this is  a very easily met  50/30milli  = ~ 1600 ohms. For ADS of just a 3 amp fuse it is a less easily met 16 ohms, and for a 32A fuse or MCB with no RCD,  forget it, go and buy an RCD.


Actually a sat cabin on its skids in a puddle probably meets the 1.6 kilohm requirement with no further actions, at least until you get a few sunny days,  but a 30mA RCD may be a bit frisky in a large installation, so you may find you have 100mA or 300mA up front RCD before splitting to each final cct. (how many cabins on the genset ?) 


I'd aim for the hundred ohm mark, not from regs, there is not one,  but because it is a number easily net in all but the driest sand, where you cannot get a shock anyway, as the ground is not conductive.

Suggest you start with driving  in a 4ft rod at the genset and if need  be another at the cabin.

calibrate your expectations and
have a laugh at some real figures measured on a cabin, a large tent and a few other things.


I was a bit surprised myself, Essex is a bit more conductive than here (Hants).

I do not understand your question. 

I would love for you to specify more (focused). Thanks

In the shed you have described there is no grounding and a potential comparison bar, so you should not reset the shed or bring a grounding shed from a reset structure. You must install a grounding electrode and potential equalizer. Shed pillars cannot be used as a grounding electrode because you do not know what goes underground and the electricity law is very clear on the subject. You must only check the grounding resistance to the ground at the above electrode before connecting it to the hollow bar and columns. You should check resistance between the potential comparison bar for each thorn. And another thing, measuring LT with a temporary feed will give you a measurement of the LT fault loop on a temporary feed and not the LT you need. Remember that LT is not a grounding measurement, it is a fault loop measurement. The best example is the reset TN-S, where the current does not flow through the ground in any condition, even in the fault. Successfully.

Hi Benyamin,

I do not understand your question. 

I would love for you to specify more (focused). Thanks

I assume from your post English is not your first language, and that perhaps you are not familiar with UK building site and wiring practice.

Let me know if the following does not make sense.

First, a site welfare cabin is a metal box/ building,  resting either on metal feet, wooden sleepers, or just on forklift pockets and skids,  as a minimum probably containing a toilet and basic washing facilities. However larger ones have can have more 'rooms' and include a small shelter/cooking/ eating area. It is dropped at a building site, or at the roadside, so that a mobile workforce have some basic facilities while mending the road or whatever. At the end of the job it is put back on the lorry and driven to the next project. electrically it may have little more than lights, or it may include a water heater, space heating and provision for microwave and kettle etc.

The poster has been asked to perform an EICR - an electrical safety inspection. He is asking what value of electrode resistance is acceptable for a mobile  generator supplying such a thing.

That will depend on the arrangement  of circuit breakers and RCD (ground fault breaker in American) that are used to protect the users of the portable building,  and what else is supplied by the generator.

Good evening, 

what's wrong?  

Even with the cabin is not made of metal as you still wrote the electricity laws do not change because there are faucets made of metal and shower shower What is not clear to you?

So, in your country, if installed to your regulations, what value of earth resistance would you require in such a case ?

Would I be right in assuming you often see problems with dry ground and high electrode resistance? Here an earth rod is typically a  straight metre or so metal bar driven into the ground, and may give 50-200 ohms depending where in the UK we are.

Paul


I asked what the rating of the generator is because BS 7430 says only generators over 10kW ( I don't know why 10 kVA is not stated) it is recommended to be connected to an earth rod. BS 7430 recommends the earth resistance to be 20 ohms.


The big questions are, what is the purpose of the earth rod? And without an earth rod what earthing system is deployed?