Insulation Resistance (Megger) testing long run cables

[W@ttson]

Hello All,

Quick clarification on standards of insulation resistance testing.

I like to do megger testing of conductors as per IEEE/ANSI ATS 2013 which says that for cables rated 600V and above to test at 1000VDC for 1 min. They also give a table to say what the minimum acceptable value is for the insulation resistance test. For example, equipment rated 600V - 1000V the minimum rated insulation resistance value is listed to be 100MOhms. However, it does list an associated distance for the acceptable value. If I test a conductor that it 10 feet long versus one that is 1000feet long the one that is 1000ft long will naturally have a higher capacitance and therefore lower insulation resistance.

IEEE 525 (Guide for the Design and Installation of Cable Systems in Substations) and IEEE 422 (Guide for the Design and Installation of Cables Systems in Power Generating Stations)attempt to quantify the effects of distance in a formula for the minimum acceptable insulation resistance. The formula is: R(in MOhms) = (rated voltage in KV + 1) *1000/(length in feet).

I do not see why NETA ATS does not have a metric to adjust the minimum insulation value for distance of conductor, and I do not want to quote substation standards in my findings of insulation resistance results of long runs. What is everyone's experience in these situations?

 

[big john]

I think it's because 100M is low for a brand new run of cable, even a very long one, so NETA considers that a safe baseline. Very often 600V cables we test are easily in the many thousands of megs.

While I agree you will have a high initial capacitance, that should charge, and the remainder of your current is leakage and polarizing current, which should very closely follow cable length as long as there's no insulation damage.

I compare resistance between circuits of the same cable type and look for values proportionate to distance. I find that's a pretty good double-check.

Remember the NETA value is also a suggestion in the absence of other standards, if I got uniformly low values, then the manufacturer's engineering recommendations are my fallback: If they say that's acceptable, so be it.

 

[Phil Corso]

... will naturally have a higher capacitance and therefore lower insulation resistance...

Spaymax...

Capacitance does play a part, but not as much as you think! The key is insulation volume and area! For example, had you tested two cables of equal length but different insulation thickness, the thicker one would require a longer test-time. Following is a brief explanation.

When testing insulation resistance, R=E/I, where E is the applied DC voltage, and I the observed current. Omitting temp and humidity factors, then, what other time-related factors are there? The observed current involves leakage paths over the insulation surface, and through its volume, resulting in three current components; one transient and two steady-state:

1. Capacitance Charging Current. Contrary to popular belief, while it starts out high, it decays relatively quickly… in the order of tenths of seconds. Essentially eliminating contribution to the 1 and 10 minute results!

2. Absorption Current. It too, starts high, but then decays over a period involving minutes.

3. Leakage Current. It is essentially a small but constant current related to both insulation volume, and insulation area.

Regards, Phil Corso

 

[W@ttson]

Ok so, brand new cables off the production line no matter the length should have no excuse to be less than 100 Mohm. Well what about a year later, 2 years later, 3.... Obviously those other standards take the length into consideration because it does make a difference when looking at older installations, but when should that formula start coming into consideration?

Also, for humidity considerations standards reference not to take measurements when the humidity is about 75% or greater, however, I have never seen correction factors for humidity, only for temperature. Has anyone ever seen some sort of correction factor for humidity?

 

[Phil Corso]

Spraymax...

So then, what you really want is more than a "Quick" clarification!

Phil

 

[big john]

There is no correction for humidity. All you can do is trend tests performed during similar environmental conditions and try to only test when equipment is above the dew point.

 

[W@ttson]

There is no correction for humidity. All you can do is trend tests performed during similar environmental conditions and try to only test when equipment is above the dew point.

Thank you, this confirms my understanding of humidity and it's role.

 

[W@ttson]

Spraymax...

So then, what you really want is more than a "Quick" clarification!

Phil

I guess it could be as quick or long as needed. I've wondered about this seemingly contradictory requirements. Or I guess I just wanted someone to say yeah sure no problem, use the formula even when using NETA ATS.

I just like using NETA ATS, for all testing, new, old, whatever it may be because I just find that it's very clearly written and that the requirements to test and results needed are nicely written.:roll:

 

[Ingenieur]

Assume 1000 vdc test voltage
1c cable laid out on earth
one lead on conductor, other staked to ground
how much capacitive coupling/leakage current will flow?

 

[W@ttson]

Assume 1000 vdc test voltage
1c cable laid out on earth
one lead on conductor, other staked to ground
how much capacitive coupling/leakage current will flow?

Depends on the condition of the conductor and on the length. The longer the conductor the greater the number of leakage paths to ground.

 

[Ingenieur]

Depends on the condition of the conductor and on the length. The longer the conductor the greater the number of leakage paths to ground.

with DC or f = 0
Xc = 1/(2 Pi f C)
so Xc goes to infinity as f goes to 0, ie, open circuit

Xc paths are in parallel Xtotal = sum (Xc's) or sum (infinity)
or if Xc is in per unit length, say Ohm/ft
Xtotal = length x Xc, but Xc = infinity so Xtotal approaches infinity, ie, open circuit

can ANY capacitive current flow from conductor to ground?

 

[W@ttson]

with DC or f = 0
Xc = 1/(2 Pi f C)
so Xc goes to infinity as f goes to 0, ie, open circuit

Xc paths are in parallel Xtotal = sum (Xc's) or sum (infinity)
or if Xc is in per unit length, say Ohm/ft
Xtotal = length x Xc, but Xc = infinity so Xtotal approaches infinity, ie, open circuit

can ANY capacitive current flow from conductor to ground?

Are you trying to prove that at T=infinity that the capacitance would be fully charged and therefore current = 0in a DC circuit? That is the case, but the insulation resistance test measures a lot more that the charging current as Phil noted. I am saying that the longer the run the more chance for constant leakage current to be present due to the increased area of the conductor run.