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Subject Topic: EHU Cable Length - What Do You Recommend?
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Message posted by davie on 02/5/2012 at 5:48pm
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Quote: Originally posted by saxo1 on 02/5/2012

The length/resistance of the earth wire won't affect the operation of the RCD it doesn't require the existance of an earth wire to operate.

Saxo1



If an insulation fault causes current to flow into, for instance the earthed metal casing of an appliance, then a RCD will detect an inbalance in the current flowing from and to the device and trip. If the earth wire is long, and thus higher resistance, then the current leakage will be less and the RCD may not trip or may be slower to trip.
An RCD will also trip if the casing (or person) is grounded - in which case, as you say, the RCD does not need an earthwire to work.

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Message posted by saxo1 on 02/5/2012 at 7:55pm
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If the resistance is too high it won't allow enough fault current to flow(30 Ma) so the RCD shouldn't trip in those circumstances. For it not to trip the current would have to be below it's rated trip of 30Ma for 200 milliseconds for  indirect contact, for direct contact 150Ma for 40 milliseconds.

If the current is below those levels it's not designed to trip..Once it reaches it's rated trip current it will trip within the required trip times, the speed of trip won't be affected so there is no danger.

Saxo1


Message posted by pepe63 on 03/5/2012 at 10:04am
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Oh?..eh.!? ..I'm glad we've cleared that up then..?!?     


Message posted by OliverDay on 03/5/2012 at 5:50pm
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Quote: Originally posted by saxo1 on 02/5/2012

If the resistance is too high it won't allow enough fault current to flow(30 Ma) so the RCD shouldn't trip in those circumstances. For it not to trip the current would have to be below it's rated trip of 30Ma for 200 milliseconds for  indirect contact, for direct contact 150Ma for 40 milliseconds.

If the current is below those levels it's not designed to trip..Once it reaches it's rated trip current it will trip within the required trip times, the speed of trip won't be affected so there is no danger.

Saxo1




Sounds like nonsense to me (when considering length of hook up) - surely the resistance of the earth conductor is similar to that of the live and neutral conductor - i.e. same length and same material. And able to carry a lot more than 30mA.


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Message posted by saxo1 on 03/5/2012 at 7:56pm
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Sounds like nonsense to me (when considering length of hook up) - surely the resistance of the earth conductor is similar to that of the live and neutral conductor - i.e. same length and same material. And able to carry a lot more than 30mA.

I'm not sure what you are saying is nonsense,I never stated that the resistance of the earth wire may be too high to carry enough current to make the RCD trip or that the cable might not be able to carry enough current to trip the RCD a previous poster stated ,Quote

"If the earth wire is long, and thus higher resistance, then the current leakage will be less and the RCD may not trip or may be slower to trip."

The statement in bold in the quotation marks is the bit that is nonsense but I refrained from pointing it as such.

.I was pointing out that if the resistance were high enough ,perhaps due to a corroded connection then the RCD would't trip anyway not that the earth wire if uncompromised would be incapable of carrying enough current.

It was suggested that if the earth wire was too long then the resistance would be higher current leakage will be less and the RCD may not trip or may be slower to trip.,(not my words) for it to be too high the the difference would need to be in Kilometers not metres probably in the region of 8000 ohms

.If the earth wire is intact then the resistance of the earth wire in a 30m hookup wuld be insignificant,even if due to a high resistance joint in the plug of 10 Ohms it would still carry 23 Amps.

I can't understand why you referred to what I had written as nonsense but ignored the obvious post that was incorrect!

Saxo1


Message posted by OliverDay on 03/5/2012 at 8:27pm
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Saxo
I interpreted your post "02/5/2012 at 7:55pm" as agreeing with what davie had stated "02/5/2012 at 5:48pm". Specifically you confirmed by stating "If the resistance is too high it won't allow enough fault current to flow(30 Ma) so the RCD shouldn't trip in those circumstances."

I then stated that that sounds like nonsense to me (when considering length of hook up). I believe that in your latest post you are now agreeing that it was nonsense with regard to a post about the length of a hook up.

I guess it would have been clearer for all if you'd just stated that davie's post was incorrect.

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Message posted by saxo1 on 03/5/2012 at 8:49pm
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If you had read my previous post you would have seen that I had previously stated

The length/resistance of the earth wire won't affect the operation of the RCD.

I was trying to explain to davie why his post was incorrect and try to reassure others that there is no risk of an RCD failing just because the EHU was long.I don't like to say somebody is wrong without attempting to explain why.The problem with electricity is there is rarely an easy way to explain it without getting too technical which appears to antagonise some posters

Saxo1


 


Message posted by OliverDay on 03/5/2012 at 9:20pm
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Perhaps you should adopt the KISS principle and then your posts won't be misinterpreted in the future.

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Message posted by saxo1 on 03/5/2012 at 10:52pm
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I  tried that method before when I stated that the amps are lower when the volts are lower and got called a bent politician and an idiot amongst other things,Perhaps I should adopt the kisfsp principle.(keep it simple for stupid people),but to do that would be insulting to the majority of knowledgeable contributors on this forum.

It appears that you recognised that davie's logic was flawed, perhaps it would have been better for you to have explained the  reason why in laymans terms,unless ther were some other reason for your comment!

Saxo1

 


Message posted by davie on 04/5/2012 at 2:02pm
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Calm down boys!
Now that we understand the physics better, can anyone suggest why
Quote: Originally posted by JTQU on 02/5/2012
There are IEE limitation on the maximum length of flexible cable that can be used in the UK; for our size wire cores it is 25 metres.
http://info.ee.surrey.ac.uk/Workshop/elecsafe/FAQ.html#Exleads


Message posted by OliverDay on 04/5/2012 at 2:17pm
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"Calm down boys!" - Thanks davie - I'm cool

The reason is almost certainly so that there are no problems caused by volt drop.

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Message posted by saxo1 on 04/5/2012 at 3:14pm
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The Regs require the maximum volt drop on a cable to be not less than 89% of the nominal supply voltage to ensure safe and satisfactory operation of electrical equipment.

If your 25mm EHU is longer than 25 metres and you drew 20A then the volt drop would be outside the regs.No real cause for concern for campers or caravanners.

The only thing it might affect could be some battery chargers if the volts dropped too low, I don't know the minimum voltage requirement for onboard chargers.

Saxo1


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