Ohms - are they a problem?

[Morganic]

I have just had my caravan serviced by an engineer who is MCEA certified. It passed but he noted the following:

"240v sockets reading 10 Ohms on Earth, should be 0.5."

I haven't had chance to speak to him yet but what does this mean and is it a problem?
What is the likely cause of this and rectification if necessary?

The caravan was serviced last year by another engineer who didn't raise problem. Caravan wasn't used after service last year so am a bit bemused as to what has caused this problem if it is one.

 

[Guest]

I ask my lad and answer coming back was, as the question is stated, it makes little sense so simply poses more questions?

He did add,you might want to have a look through http://www.electriciansforums.co.uk but I'd get more info of your engineer first

 

[Martin_E]

I think your engineer may be confused,

the figures are............

• 
  0,5 Ω protective bonding,
  1,0 Ω protective earthing ,
  
  So the figures he has given you are for protective bonding (bonding to chassis, gas pipes etc) and not for protective earthing (sockets etc)

 

[ProfJohnL]

Hello Morganic,
You have not provided enough detail information about the discrepancy your tester has identified. Consequently its impossible to offer any specific advice. What I can say is that assuming the tester has carried out the work properly, then the system should be safe, but I have to say it suprises me he/she should pass a system, and then point out a major discrepancy, so the best thing is to ask him/her to explain in detail.

 

[Damian]

Without wanting to pillory anyone in particular,but this type of scenario is exactly what I thought would happen with the electrical testing setup as advocated by the NCC AWS, and adopted by the MCEA.
You have people who have no real in depth knowledge of electrics being taught the very basics and then not giving the correct information, in its correct significance, to an owner.
I have to say that unless one has a background in electronics, then the two day courses run by the NCCAWS are not sufficient to impart all that one needs to know and how to apply results. You cannot compress three years or more of learning into two days!

 

[Guest]

Could not agree more Damian and that would be something else my son would have said had he not been talking to me?,

He's not long passed his third and final exam part P old wiring, that after 2-3 years and thousands of pounds in lost earning/fees plus buying the requried kit....a little knowledge is a dangerous thing?!

Passed as in not got the final figure yet, but the other two apprantly were highest recorded scores at his centre!..... not saying I'm glowing with pride or anything!

 

[Morganic]

Thanks to all respondents
Have now more info. The problem lies between EHU input socket and 240 v sockets. I suspect terminals are corroded on EHU input socket.

 

[RichardJ]

The readings will confuse anyone, 10 ohms, from where, if the reading is from either the live or neutral then there is a problem, that is a short circuit, the readings from l or n to earth should be infinity, between N and L should also be infinity, the point of testing LNE is to prove there are no circuits between, therefore no potential dangers.
if the problem is there, which it doubt very much, it's either a faulty appliance still plugged in, or damp!, it's a million to one shot the resistance from either neutral or live to earth is only 10ohms.

 

[jignc]

I had a similar problem only my socket earth was non existent ie open circuit.
I found the earth pin on the wiring loom inline plug and socket was not connected and never could have been as the wire to it was not long enough for the female end to reach the male pin.

This was after two full annual services. The way each socket is connected using a very flimsy three pin inline flat plug and socket is asking for trouble.

Check the inline plug & sockets.

 

[ProfJohnL]

Unfortunately as often happens on open forums, the information we have been given is insufficient to draw any secure conclusions.

So we still don't know exactly what has been measured. What we should be able to rely on is that if a technician is properly qualified, they should be able to check and if satisfactory, certify the electrical installation. What I find difficult to reconcile is that after certifying the installation, he/she then tells the owner there is a problem.

The subsequent information "The problem lies between EHU input socket and 240 v sockets" is still leaves many unanswered questions. It also leaves the suspicion the techi may not have done the certifcation properly.

 

[Morganic]

Sorry it's the way I posted the thread.
Everything else passed during the service except this ohm problem. Seems when he checked the earth continuity from hook in point to 240 sockets is where he found a problem. So the problem lies somewhere in the caravan circuit.
The van is not old so having discussed it with him he concludes resistance may be due to poor contact on input socket pins or indeed could be in EHU cable used in test. We'll see what outcome is after he has cleaned input socket pins and continuity of hook up lead.

 

[Morganic]

Sorry it's the way I posted the thread.
Everything else passed during the service except this ohm problem. Seems when he checked the earth continuity from hook in point to 240 sockets is where he found a problem. So the problem lies somewhere in the caravan circuit.
The van is not old so having discussed it with him he concludes resistance may be due to poor contact on input socket pins or indeed could be in EHU cable used in test. We'll see what outcome is after he has cleaned input socket pins and continuity of hook up lead.

 

[Damian]

Quote " he concludes resistance may be due to poor contact on input socket pins or indeed could be in EHU cable used in test."

Sorry, but he should have tested the EHU cable and zeroed his test meter to the EHU cable before starting any testing. He also should have ensured that the pins and sockets were clean and in good condition before testing.
It seems as well that he really has no idea what he is doing,and is costing you money which you need not be paying out for.
If, in his words, "it could be poor contact on input pins,", then how has he managed to rule out possible loose connections on either the EHU cable, rear of the input pins, inside the distribution box,at whichever socket has the "problem",,,,,,,,,.
He has, as said earlier, created more questions than answers.
What code did he put on the report sheet? as if there is a fault, it should have a code number.

 

[xtrailman]

I didnt get "pat" testing training when i worked as a maintenance electrical technician, the day electricians did that part of the job.
But around 40 years ago when i was an electrical contractor i did a short spell testing installations, coucil homes, Marks and spencer stores.
Back then the effectivness of the earth was tested with a gadget that plugged into the mains and came with a probe, this was then touched onto any metal part to test the resistance to earth, it also put around from memory 15 amps through the circuit.
I found a few missing earths, as in no wire atall, and a couple of connections that failed the current test.

 

[Martin_E]

I agree 100% it sounds like half hearted testing with no idea what should be done.

Quote " he concludes resistance may be due to poor contact on input socket pins or indeed could be in EHU cable used in test."

Sorry, but he should have tested the EHU cable and zeroed his test meter to the EHU cable before starting any testing. He also should have ensured that the pins and sockets were clean and in good condition before testing.
It seems as well that he really has no idea what he is doing,and is costing you money which you need not be paying out for.
If, in his words, "it could be poor contact on input pins,", then how has he managed to rule out possible loose connections on either the EHU cable, rear of the input pins, inside the distribution box,at whichever socket has the "problem",,,,,,,,,.
He has, as said earlier, created more questions than answers.
What code did he put on the report sheet? as if there is a fault, it should have a code number.

 

[ProfJohnL]

Hello Morganic,

As I have already pointed out we are not in possession of all the facts, BUT based on what you have eventually told us, I am very concerned about the competency of the technician who has told you there is a high resistance problem on the earthing side of the caravan. The electrical safety of any installation in the UK relies on good contacts and conductors with low Ohmic readings and insulation with high Ohmic resistancees. Out of range values of any of these compromises the safety of the system and should record a fail with no certification.

 

[Woody3]

In simple terms, if you pass one amp of current through a resistance of one ohm the voltage drop across the resistance will be one volt.
For domestic installations the resistance from any 13A outlet to earth must not exceed 0.4 ohms under certain conditions, and (IMSMC) 0.15 ohms normally. In a caravan where the cable should be fairly thick and short, the electrician is telling you that the resistance should not be greater than 0.5 ohms (which does seem a bit high) and your caravan has a resistance of 10 ohms.
It may be a surprise, but 10 ohms is actually very difficult to achieve. I wonder if he misread it and it was actually 1.0 ohms - which can be achieved with a corroded or oxidised connection. A loose screw on a connection would not usually be enough. If he was using a PAT tester then there could be quite a few amps of current being passed through the earth connection and a loose screw would cause serious heating and most probably a smell.
As others have said for a full and correct answer we would need to know where the test was being performed: did it include the EHU cable (I would have though not) or was it from the incoming mains connector to one or more sockets in the caravan? We need more info.

 

[Garfield]

."For domestic installations the resistance from any 13A outlet to earth must not exceed 0.4 ohms under certain conditions, and (IMSMC) 0.15 ohms normally"

What part of 17th edition is this?

If the tech was doing an earth loop test and it was a TT (earth rod) supply, then 10 ohms is actually quite good. Assuming that the caravan and / or the mains supply are RCD protected, then 10 ohms is not a problem, (100 ohms is allowable in a TT domestic supply loop test). High resistance in the earth cable will not cause overheating in an RCD protected circuit (assuming RCD is working) as the earth cable will never carry more than 0.03amps for more than a few thousandths of a second.
Bottom line is that, without knowing the exact test done, a definitive answer cannot be given (having just opened my big mouth!)
The correct way to test the earth R1 +R2 resistance in a caravan and EHU lead would be to short the live and earth pins on the plug end of the lead (with the other end plugged into the caravan) then measure resistance between live and earth at the sockets in the caravan ( I cant be bothered to trawl through my 17th edition to find the limiting values but I would expect somewhere in the region of 1-2 ohms),
Alternatively measure earth loop impedance at the mains supply socket then connect the caravan and perform the same test on the caravan sockets, subtract first from last results to get the answer

 

[Garfield]

How do you do paragraphs on this forum!

 

[Gagakev]

How do you do paragraphs on this forum!

You dont!!!!!!!!!!!! The sit is broken and the powers to be (Haymarket) cant be botherd to fix it, they keep promising a new site but thats been going on for a couple of years now, (I might have exaggerated but cant be far wrong !!!!!!!!!!!!!!) Since it went wonkey IMO the traffic on hear has declined.

 

[Damian]

I do not know if you have the same screen as me, but if when you post your reply you have the facility to edit your post afterwards, then click on Edit then scroll down to the bottom of the posting and if you have a button marked Input Format,click on that and select FULL HTML instead of Filtered HTML and the paragraphs will appear by magic. Then Save.

 

[Guest]

I've had this problem since Christmas and wondered why my posts were now devoid of paragraphs? It now seems someones changed the default HTML settings!!
Would it not then be a good idea to get said someone to reset the defaults and then leave things well alone???

 

[ProfJohnL]

<p>With great difficulty, Its an absolute pain. .................................................................................................................................................................</p><p><br /><br />To comment on your main reply, You have come to the same conclusion about the quality of the information we have received. Its insufficient to pass a technical opinion on. </p>

 

[ProfJohnL]

I do not know if you have the same screen as me, but if when you post your reply you have the facility to edit your post afterwards, then click on Edit then scroll down to the bottom of the posting and if you have a button marked Input Format,click on that and select FULL HTML instead of Filtered HTML and the paragraphs will appear by magic. Then Save.

Hello Damian, Parksy has also suggested the same full/filtered switch, but I certainly dont have that option using chrome/Firefox or IE, and others have reported the same as me. It seem likely the God's Moderators package has the HTML tools and us mere mortals don't. I'll bet the software developer who has made the changes to the current package didn't actually test the user interface from the user side of the application. They probably have the same suite of tools the Moderators have, and thus is unaware of the difficulties users are actually having. I did not join this forum to take an advanced course in HTML.! I have enough difficulty with English!.