Test Cable Ratings

[paul8f]

Anybody know why an Electrical Safety Tester (Insulation Test device) with a max O/P of 5kV has the following cable ratings:

Red Lead... 10kV dc, CAT III.
Black Lead... 1kV.

Just wondering if there's any reason why it comes with cables that have different voltage ratings?

[DTJ]

Just guessing that the -ve output of the tester is connected to the testers chassis and mains earth so it's got no where to leak to.

[paul8f]

Just guessing that the -ve output of the tester is connected to the testers chassis and mains earth so it's got no where to leak to.

Yeah, sounds plausible.

But, what about the following scenario...
  (1) The tester is set to its max output of 5kV,
  (2) There is very low resistance presented between the Red and Black test crocodile-clips (e.g. due to faulty DUT),
  (3) There is a poor connection on the negative test lead's banana plug that connects back into the -ve terminal of the tester.

In this unlikely, but possible situation, very little voltage gets dropped across the faulty DUT as it's almost a dead short, so most of the 5kV gets dropped across the high resistance joint of the banana plug due to a corroded/frayed wire or poor terminal mating etc. This means that 5kV will be present along the whole length of the Black cable, which is only rated for 1kV.

Any thoughts?

[LazyJack]

It's not about the voltage the tester gives out, it is current limited anyway. It is the tested device or circut that may be energized. Otherwise the CAT rating doesn't make sense.

[threephase]

I would be curious to know the make and type of the safety tester you are using, any of the high voltage testers that I have used at 5kV are battery operated and have the same rating for both test leads. At higher voltages and mains operated, there can be different types of leads for the +ve and -Ve connections.

A lead in itself, does not have a CAT rating, that is the rating of the input of the tester that has the capability to read up to 600V / 1000V, to warn against an energised circuit.

The output of the test set will be isolated, so the potential difference can only be seen across the red and black lead, so in the event of your scenario, the black lead only sees 5kV with respect to the red lead, that is rated at 10kV insulation.

If you connected the test set up with +ve to the chassis of the DUT and the -ve to the winding, then under your scenario, the black lead could be stressed. That is why high voltage test engineers are careful about how they place the test leads and connect to the DUT, to ensure that there is little risk of leakage between leads. The guard terminal on the tester can also be utilised to avoid issues with current leakage.

Depending on the type of test set, the output current will be limited to some extent, generally maximum of 15mA, but usually much lower for battery operated instrument. When the current goes above this, the output voltage of the tester will drop, so there is a low risk to personnel carrying out the test.

The situation you have presented would also be due to poor working practices. In industrial environments where these kind of testers are utilised, there will be strict control measures in place for carrying out this kind of testing, and one of those will be inspection of the test apparatus prior to carrying out the test.

High voltage testing of any kind will always carry certain risks with it. That is why the test engineers are specifically trained and follow strict working practices.

Kind regards.

[DTJ]

The situation you have presented would also be due to poor working practices. In industrial environments where these kind of testers are utilised, there will be strict control measures in place for carrying out this kind of testing, and one of those will be inspection of the test apparatus prior to carrying out the test.

High voltage testing of any king will always carry certain risks with it. That is why the test engineers are specifically trained and follow strict working practices.

Kind regards.

At a company I worked at we set up a 'fenced' area with a physical barrier. Equipment to be tested was wheeled in and the gate closed before any testing could be done.

[paul8f]

I would be curious to know the make and type of the safety tester you are using

Thanks for the input. It's a SEW 2803 IN. It's a portable battery operated unit, and also has a green Guard cable.

The output of the test set will be isolated, so the potential difference can only be seen across the red and black lead, so in the event of your scenario, the black lead only sees 5kV with respect to the red lead, that is rated at 10kV insulation.

The scenario I described is kinda the worst case scenario! When the -VE and +VE cable clips are shorted together, the -VE cable effectively becomes an extension of the +VE cable. You made a good point questioning what the high voltage was in respect to..... In my scenario, wouldn't the 5kV be the PD between the end of the black cable, and where it meets the faulty banana plug? If so then, if arcing were to occur anywhere, it would be here at the break in the cable near the -VE banana plug.

In a scenario where a fully functional tester is used in a normal situation (i.e. with no faults), then the 5kV would be as you described....  the potential difference would be between the red and black cables (the red one having the 10kV insulation rating, acting in a way like insulation for both test cables.) Thanks for pointing that out!

Am I correct in saying that the guard feature is basically a secondary -VE cable but running parallel to the +VE cable as a co-axial screen?  This is probably another reason why the tester doesn't need a 10kV rating on the black cable.

[threephase]

If there was a full break in the black lead, then yes, there would be a 5kV potential across the break, so the insulation on the black lead at that specific point would be over stressed. Potentially you would get arcing at the break in the cable, but the test sets have very low energy, so the arcing becomes intermittent. If you used an insulation piercing probe and went along the black lead then you could measure 5kV with respect to the -ve terminal along its length, but not with respect to the DUT or the case of the DUT.

We get similar effects when we test insulated bearings and do not obtain enough clearance between the rotor shaft and case, you tend to hear it more than you can see it, unless you are in subdued light.

You are right that the guard terminal is usually held at -ve. Some manufacturers will shield the +ve lead and others the -ve. The guard terminal can also be used with a separate lead as a 3 wire test to prevent leakage affecting a reading. So if I wanted to get a true reading of the insulation between the primary and secondary of a transformer, I could connect +ve to primary, -ve to secondary and then guard to the transformer case / core. Then any leakage from primary to case back into secondary is ignored and I get a true reading of the insulation from primary to secondary.