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LV Cable

01/30/2025 10:47 PM

We have an ongoing study of LV cable which is installed 25 years ago and still functioning as intended. The cable/s is/are supplying motor/s via VFDs.

Now, we have an ongoing project the need to replace the existing Pumps and Motor as well to a more bigger capacity.

IR was conducted and its reading is 18 Gega Ohms.

Based on the IR reading, what is the status of the LV cable?

With reference to the IR reading, Is it still possible to use the existing cable, in which its size is still capable to supply the new motor?

With reference to the IR reading, do we need to replace it with new one?

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#1

Re: LV Cable

01/30/2025 11:05 PM

Gibberish..replace the wires...

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#4
In reply to #1

Re: LV Cable

02/01/2025 5:50 PM

Can you please elaborate further why we need to replace the wire?

Thank you

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#6
In reply to #4

Re: LV Cable

02/01/2025 7:21 PM

Well assuming IR stands for insulation resistance test, and you are testing at standard, your ohm reading is too small, a minimum of 25 M ohm is required and testing at 2.5kv is spec...That is unless you meant giga ohms, in which case the wire tests good, and can be used....Gega seems to be a cross between giga and mega so which is it...?

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#13
In reply to #6

Re: LV Cable

02/09/2025 10:49 PM

it is 18 Giga Ohms (GΩ).

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#14
In reply to #13

Re: LV Cable

02/09/2025 11:20 PM

There are many good and bad reasons why someone would not want to change a motor cable. What is your reasoning for not replacing this cable?

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#12
In reply to #4

Re: LV Cable

02/05/2025 9:59 AM

A qualified local Electrician can do that, on the basis of condition and on a calculated need to resize or not as the case may be. That's what this trade does as a matter of routine.

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#2

Re: LV Cable

01/31/2025 6:29 AM

Of course, it is possible to reuse these cables but where is the advantage in not replacing them? You have not provided a valid reason for not replacing them.

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#3

Re: LV Cable

02/01/2025 1:36 AM

Rather than some unknown method of insulation resistance test, why not investigate the properties of the insulation and determine what the expected life of the compound is.

Also, dielectric absorption test may be more definitive, though not usually useful for low voltage.

If you would replace it in kind, and it would not suffer in the reconnection activity, then economically it makes sense to reuse it.

For example:The lifetime for XLPE is expected to be between 40 and 60 years at 90 °C rated operating temperature. Experimental findings of this study show an estimated cable lifetime between 7 and 30 years for rated operating temperatures between 95 and 105 °C.

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#5
In reply to #3

Re: LV Cable

02/01/2025 6:00 PM

The lifetime for XLPE is expected to be "between 40 and 60 years at 90 °C rated operating temperature":

1. Is this industry standard acceptable practice?

2. Are there International Standards that specify that the cable should be replaced after certain years, say 25 years?

3. What "time span" the cable manufacturers recommendation for LV cable to be replaced?

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#7
In reply to #5

Re: LV Cable

02/01/2025 10:56 PM

You posted this as off-topic, but I don’t think it really is.

This is your installation, why would you need external validation for something you can evaluate on your own with facts?

It is a value decision that can be easily made if you have all the facts. Or you can present the facts to the decision maker.

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#8
In reply to #5

Re: LV Cable

02/01/2025 11:17 PM

The life of the cable insulation is determined by its environment...that's why you test it on a regular basis....bug eat it, lightning hits it, water ingress, mechanical failure, traumatic intrusion, animals attack it, the sun degrades it, bacteria, fungus, molds you name it....

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#9

Re: LV Cable

02/03/2025 7:30 AM

<...its size is still capable to supply the new motor?...>

Cable sizing has been discussed to death on CR4.

Simply follow the wiring code applicable to the country of installation, such as British Standard 7671. If in doubt, consult a qualified local Electrician; these people routinely size, install, test and certify as safe installations, both existing and new ones.

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#10

Re: LV Cable

02/03/2025 8:19 AM

Many questions:

  1. Firstly, can the existing cable handle the increased load from the VFD/motor/pump combination? (Increase in current, decrease in power factor or longer cable run may require larger diameter cables.)
  2. I'm assuming the voltage is staying the same?
  3. Is the duty cycle the same?
  4. What is the running temperature at the moment (i.e. is it running close to the max temperature rating)? Have you done infra-red thermal sensing on the cable run, joints & terminations?
  5. Are you keeping the VFD and not reverting to (say) star/delta or something else?

If the distance is not too long (i.e. the additional expense would not be that large), it may be worth doing it during the planned downtime.

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#11
In reply to #10

Re: LV Cable

02/05/2025 9:13 AM

A1) BS7671 and its application can answer that one.

A2) assumptions involve risk.

A3) BS7671 is for 100% <...duty cycle...>.

A4) BS7671 and its application can answer that one.

A5) BS7671 and its application can answer that one.

<...If the distance is not too long...>

What does this mean? BS7671 embraces cable length as part of its protocols for cable sizing.

The recommended course of action remains for the Original Poster to engage a qualified local Electrician.

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#15

Re: LV Cable

02/11/2025 8:52 PM

IR reading of each cable:

MotorVolt/Phase/CyclePhase A to Phase BPhase A to Phase CPhase B to Phase CPhase A to GroundPhase B to GroundPhase C to Ground
A460V/3/60Hz38.6 GΩ36.8 GΩ44.3 GΩ22.4 GΩ22.7 GΩ23.9 GΩ
B460V/3/60Hz38.1 GΩ42.2 GΩ38.4 GΩ22.3 GΩ23.1 GΩ22.8 GΩ
C460V/3/60Hz9.9 GΩ13.7 GΩ10.88 GΩ6.23 GΩ5.9 GΩ8.81 GΩ
D460V/3/60Hz35.5 GΩ40.5 GΩ43.6 GΩ21.2 GΩ21.4 GΩ22.9 GΩ

On the above results, base on your opinion and experiences, what could be the reason why Motor C has the less reading compare to other 3 motors.

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#16
In reply to #15

Re: LV Cable

02/12/2025 12:32 AM

A sweaty handed electrician handled the wires during installation. I would reuse each and every one of those conductors without reservation, based on those tests. Assuming a 500V or 1000V dc test is producing those resistance values, my 40+ years of practical heavy industrial experience has not failed me, including 600+ volt PWM inverter drives up to 400HP in some cases.

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#17
In reply to #15

Re: LV Cable

02/12/2025 3:02 AM

Another possibility is that the cable to motor C is 4 times the length compared to the others though it cannot be seen from here.

Remember that unless a pass/fail criterion has been determined before work starts, all one has is a measurement; it isn't a test.

Are these the new cables designed for the higher loads, or the old ones on the existing loads?

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#18
In reply to #17

Re: LV Cable

02/12/2025 3:05 AM

All cables were almost have the same length of 110 meters from VFD to motor.

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#19
In reply to #18

Re: LV Cable

02/12/2025 3:07 AM

At last!

  1. What was the outcome of the Electrician's calculations on the cables for the proposed increased loads?
  2. Is the upstream supply adequate?

Worry about those things first.

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#21
In reply to #18

Re: LV Cable

02/12/2025 11:09 AM

You have finally provided relevant additional information. Four runs of 110-meter-long cabling can be expensive to replace. Still, a qualified individual (electrician) needs to know much more to assess whether to replace these motor cables. The insulation leakage test results are one such criterion for replacing cables and the posted results are probably adequate for the job.

Hire an electrician and heed their advice.

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#20
In reply to #15

Re: LV Cable

02/12/2025 3:46 AM

It's amazing how the reciprocal of 0 can vary so much.

Did the test unit that was used report pass or fail (BS7671 part 6) on these figures?

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