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Topic Title: LV Distribution Cables with Low Insulation Resistance
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Created On: 14 November 2012 09:18 PM
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 14 November 2012 09:18 PM
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TeesdaleSpark

Posts: 660
Joined: 12 November 2004

I have a client that has lots of LV distribution cables burried underground with low levels of insulation resistance (some lower than required by BS7671). It would be very expensive to replace them and he wants to know if there is any justification for them remaining in service.

What do DNOs do when their LV cables have low insulation resistance?
 14 November 2012 09:22 PM
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slittle

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I suspect wait until they go bang given the costs involved with highway works.


Stu
 14 November 2012 11:22 PM
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rocknroll

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In my very brief period of engineering with the MEB a long time ago these cables (distribution) did not have the same tests as outlined in BS7671, it also depended on the cable, paper, lead, XLPE etc;

A hi-pot tester was used and it was generally 1000v @ 1 minute, it was not uncommon to see old cables as low as 10Mohm and this was regarded as safe for energisation.

There was also a outer sheath insulation and screen test at a higher voltage to determine the quality of the sheath, maybe one of the power guys can give you details of the current tests that are done and with what instrument.

I agree with Stu they will probably run things until they self-destruct and then dig the road up. LOL

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------

Edited: 14 November 2012 at 11:38 PM by rocknroll
 15 November 2012 02:23 PM
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broadgage

Posts: 1246
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Yes, DNOs wait until they go bang.
In most cases though they cant measure the insulation resistance of existing LV cables as most have numerous house services and street lights connected to them, which cant realisticly be isolated.

Some LV cables simply link one transformer to another, or feed a single relatively large consumer, these can be tested and sometimes are.

As regards privately owned cables, I would consider replacement if the insulation resistance is very low, say less than 1Mohm.
A reading that low suggests a significant risk of sudden failure, with presumably significant impact on the customer.

If the reading is better than 1MOhm, but still lower than accepted values, then I would monitor it regulary and replace the cable if it gets much worse, but not otherwise.

All this presumes relatively short cables of course.
A low insulation resistance is to be expected on very long runs, since the leakage currents add up.
Consider a cable one tenth of a mile long, with a reading of 100 MOhm, entirely normal. Now consider 10 miles of the same cable, a reading of only 1Mohm would be expected.
 15 November 2012 02:37 PM
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rocknroll

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A reading that low suggests a significant risk of sudden failure, with presumably significant impact on the customer.


LOL Do you really think so, I am aware of many many occasions where the IR was <0.5 and the distribution cables were left in service and probably are still now, my experience refers back to a group of feeders from the station to the Bull-ring that were well below this figure and the decision at the time between our group and the engineers was to leave them in service and I bet they are still there now leaking plenty of A's into the ground as all distribution cables do. As we were in the transition stage between public and private the major expense and disruption to rectify this was not practical as it would knock a massive hole in the budget that was needed elsewhere.

regards

-------------------------
"Take nothing but a picture,
leave nothing but footprints!"
-------------------------
"Oh! The drama of it all."
-------------------------
"You can throw all the philosophy you like at the problem, but at the end of the day it's just basic electrical theory!"
-------------------------
 15 November 2012 02:59 PM
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OMS

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As a starting point, most DNO's require new installed cables to achieve not less than 50 megohms at 500V.

In practice however, they will run thier systems with very low IR values and in some cases, I'd concur with our resident sailor that even down to 0.5megohm would be OK to use

In a number of global standards, 0.5 megohm is used as the minimum value for continued service of cabling (tested at twice the line to earth voltage or 500V for 400V systems.

Very low IR may be an indication of "approaching failure" as broadgage mentioned so your client needs to keep that in mind.

I assume the measured values you have were taken shortly after the cable was put out of service for testing rather than after a period of being de energised.

Regards

OMS

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 15 November 2012 04:27 PM
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Delbot321

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Joined: 06 November 2012

Having worked on DNO systems they expect high readings on new cables but once in service with lots of users jointed to the cable down the street it is not re-tested. If it fails and short circuits they will usually put larger and larger fuses in the feeder piller off the transformer to blow the fault clear assuming the cable is undergrount as the explosion/burn of the cable is not likely to injur someone. They also use cable testers but their use is limited due to the amount of T joints off of the cable. It is not uncomon to see smoke or steam out of the ground where the fault is. Once the fault has blown clear it is easy to find the fault along the length of cable by working out which customers are on and off and using a CAT to trace it. Overhead they will visually look for the cause.

Going back to your cables underground - the previous advice seems a suitable approach - advise client it can be sign of breakdown but also based on risk assessment what are the consequences of a failure. Inconvienicen, loss of service etc. Is there any likelyhood of injury or death due to a failure? Cable could go for years as they are - if they do develope a fault you can usually use cable testers to track the fault to within a couple of meters, dig up that section and replace it and on you go for many more years.

However you also need to consider if there are joints along the cable? Is the armouring/earth providing a sound fault path? Have the earth loop impedances been checked at the load ends of the cables? Is it providing the required level of shock protection? What also happens is people put more and more loads on to it and the volt drop exceed that permitted and equipment starts not to funtion?

If it is a single cable from end to end testing is quite easy, if it has underground joints with cables T'ed off then this is a whole lot more difficult.

A few more thoughts for you
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