|Product Performed to Expectations:||10|
|Specifications were sufficient to design with:||10|
|Demo Software was of good quality:||8|
|Product was easy to use:||10|
|Support materials were available:||10|
|The price to performance ratio was good:||10|
|TotalScore:||58 / 60|
This review is the first part of my review on the Keysight U1461A Insulation Tester.
(see the second part here )
photo source Keysight
Fortunately Peter Oakes, Gough Lui and I have all chosen to focus on different parts of this meter's abilities, so the intention is to link to each others reviews.
In my application I was interested in using it for testing communication cables and the Line Protection modules that are fitted to MDF (Main Distribution Frames) installations.
What are my first impressions?
Despite looking at the brochures before applying, I had not fully comprehended what a versatile and useful device this meter is.
The ability to reduce the test voltage down below the normal 250v was very useful, but it has far more features than that, and in fact was easier than I first imagined.
At work when I demonstrated how easy it was to test Line Protection units, they decided to add this to the next instrument purchase.
I suspect our department won't be the only one, and they may replace the other meters we purchase, given their multi role capabilitites.
What is an Insulation Tester?
It became apparent that many people are confused about what an Insulation Resistance tester is, and what it does.
In order to ensure that people are safe from accidental contact with voltages greater than 32vAC, it is important to have something that can check this.
Since most mains appliances run at either 110 or 240v AC, you need to apply at least that voltage to the various parts of an appliance to check its safe to use.
The theory being that if it handled twice the voltage (500vDC), then it was safe (rather like a bridge has an element of overload in the design, otherwise it could fail).
The test instrument used is an Insulation Resistance tester and in the past this consisted of a meter with a hand crank to generate 500v DC (or 250 for other parts of the world) which was applied to the device in question.
They were generally referred to as a Megger, and I never knew any other brands.
We spoke about this during the application discussion, but an old trick was to get the new trainee to hold on, while you cranked it.
Since they knew a little bit about electricity, they realised it would hurt and were "... apprehensive ..", until shown by a more senior staff member.
What we never told them was to wet their fingers and hold on REALLY tight, which tends to reduce the skin resistance and means the voltage is very little, hence no shock.
I've got an Ohm meter what about that?
While modern ohm meters will measure very high resistance, they usually run from 6 or 9 v and have no way of generating the 250/500 or 1000v needed to test insulation breakdown.
To give an example, a few months ago my water pump stopped and there was no power to the outlet.
The HRC fuse had blown, and I checked with a meter and could find no shorts, and the pump appeared to be okay.
We've had power surges before on a single phase, but since nothing else appeared to have stopped, it was strange it was only the pump.
I had another fuse (one only) and decided to do an IR test on the pump and lead to be sure.
It turned out the lead to the pump was breaking down, and had suffered from moisture over the years.
While it was okay when dry, the recent wet damp weather had caused the insulation rating to fall enough to add to the load and cause the fuse to blow.
Had I simply replaced the fuse, it might have gone for a while, but blown at the worst time (damp, cold morning when you're late for work and need a shower)
What causes insulation breakdown?
There are many causes, but in the case above, the plug and socket both had mould buildup inside.
The pump and wiring are in less than ideal conditions, and suffer from heat, cold and dampness, so its understandable that when it gets wet it could conduct and reduce the insultation rating.
Since it had been in place for 12 years, there was no immediate requirment to do anything but replace the cable, plug and socket and retest to make sure.
Another cause is overload damage which can cause pinholes in the insulation to earth or other wiring.
This may be as a result of damage to the insulation from poor assembly or abrasion while in service.
It might have been okay when it was manufactured but you now have it and you last worked on it, which moved the wire, and suddenly there is a problem.
The insulation may have cracked from old age and brittleness, excessive heating and cooling, or even vermin as the photo below shows.
Poor construction or inadequate checking during manufacture amy also contribute to later failures, as this site shows.
I am aware that the internal roof temperature of some modern homes was exceeding the temperature rating of some mains cables, resulting in their premature breakdown.
Just like mechanical items, age can cause things to fail, so it's important to check the Insulation Resistance when you service something.
In Australia and New Zealand it's mandatory for anyone doing this commercally (for payment or reward ... that would include beers).
I was reminded of this last year when I was asked to troubleshoot some issues on a rare and expensive car that had been rebuilt.
The car had sequencing tailights and because the original electromechanical units are not longer available, electronic equivalents were fitted along with an electronic flasher.
In digging around to find where they had placed this faulty flasher unit, I noticed the wiring loom had been incorrectly routed under the dash and was rubbing on a metal bracket.
What was more disturbing was it had already damaged several wires which had bare copper showing.
Luckily I had noticed it, contacted the owner and rectified it, as I could have been the last one to work on it before a very expensive bonfire.
So if you intent to fix things that plug into the mains, do yourself a favour and buy an IR tester, and use it.
What about these other 'things' you mentioned.
Communication cables and other cables like Cat5 or 6 do not require to have an insulation rating for mains voltages.
Depending on the brand of Ethernet cable the rating might be as low as 50v.
Belden (and others) make a cable with 600v rating for use in industrial control in switchboards, etc.
I don't think you'll find these in everyday use.
The UL listing for many of the markings is here.
Some POE system use up to 57v, and I saw a few Cat5 cables with 125v ratings.
Since most phone systems use 48v DC, you can assume communication cables should handle 100v DC.
Obviously if you apply 500v DC from your standard IR tester, you're either going to get a pass or punch a pinhole through the insulation depending on what is connected to the other wires.
This pinhole may stay there for some time, until finally the wire right next to it changes polarity and is an earth, or has some other control on it, then your 'holed' conductor is a problem.
One of my work collegues had this very thing happen on a brand new installation using a new cable.
Testing a communication cable
The ability to reduce the test voltage to 50v (or set it down from 10-50) means you are unlikely to punch through the insulation, and this provides a go/no go test.
BUT this meter has some other clever tricks.
You can set it to DAR (Dielectric Absortion Ratio) where it measures the insulation resistance at 60 seconds v testing at 30 seconds (or you can set to 15 secs).
Every cable will slowly charge up as you apply a DC voltage. The effect is caused by capacitance.
Whenever you apply a DC voltage to a capacitor, the current drawn dimishes as it charges up, and this test allows that effect to be checked.
Obviously if a cable has resistive leakage, then the ratio is not going to be as good (or constant if low enough).
This site suggests that anything greater than a 1.6:1 ratio is good.
Polarisation Index is the ratio of the Insulation Resistance at 10 mins v 1 min.
The purpose of this is to allow the electons to align themselves and represent their working state.
This takes about 10 mins, while the capacitive effect (seen in the DAR test) disappears after 1 min.
The above linked site suggests a PI result above 4 is excellent.
The idea of both of these tests is to ascertain where the problem is, and is probably much more relevant in High Voltage Cables and Motors.
I found some interesting reading in these links, and I'm sure there are many more.
What is clear is that it isn't a simple go/no go test as this picture shows.
The other useful test that can be applied is an increasing voltage, which very clearly shows a poor resistance as the voltage increases.
The voltage is applied for the same time at each step, and the default is 10 seconds, but can be user configured.
In order to prevent damage, the trip current can be set from 0.01mA to 1.5mA, which will stop the test.
This step test is what I used to test the MOV's in the Line Protection unit.
What about the Line Protection units
These devices are designed to fit to a Krone Block MDF, and are intended to protect equipment from harmful voltages derived from external sources (ie lightning)
We use them at work on all communication cables entering/exiting the building, and in most cases they prevent further damage.
BUT we had no real way of testing them easily and safely.
These protection modules come in diferent voltages depending on the manufacturer.
Using the step voltage test made it quick and easy to verify which ones were faulty and the extent of the damage.
I left the current at 1mA and set the scan time for 5sec.
I had trouble finding the exact specs for these, but suffice to say the good ones test 247v across the line, and greater than 500v to earth, while the two damaged ones measure 27v and 3v3 across the line and greater than 500v to earth.
As a comparison using the 250v IR test just gave a fail on both good and bad, with no indication of what voltage.
This is all we had in the past to test these devices, so consequently the field techs have to just replace them and hope they were the cause.
In the next part I'll cover some of the other features useful for these tests.