Type J TC Voltage Rating

[Bob O 8]

I have been searching for the voltage rating of type J TC extension wire [White/Red] this morning without any luck. The customer wants us to megger these and they don’t know what the rating is for their existing TC wire. Any help is appreciated. Thanks, Bob O. Edited 30 May 2009 by Bob O

[Rodney 21]

Bob O Try the website below you are looking at Mv (EMF). http://www.omega.com/techref/thermcolorcodes.html Rodney

[Bob O 8]

Rodney thanks for the reply. I searched omega before posting but didn't find the info I was looking for. Example THHN is rated at 600V so we test at 1000VDC and they have other cable that is rated at 300V and we test that at 500VDC but the TC cable isn't marked and they don't know what it should be tested at. Thanks Again,

[Rodney 21]

Bob-O What is the reason for your customer wishing you too megger the J type extention cable. This cable should aways be run on its own as not to pick up any interference from any voltage cable. Rodney

[Bob O 8]

It is in its own pipe and there spec is to megger every conductor into the panels. I am going to assume they are wanting to check if any of the conductors were damaged during install. Thanks Again

[Mickey 71]

Then use a Ohm meter.

[paulengr 44]

There is not a straightforward answer to your question. The standard answer is not really relevant. I'm going to give you an answer but in a roundabout fashion as we walk through what would be an appropriate specification, and why, because there's not a text book / specification answer here. First why use a megger? Meggers are just like high range conventional ohm-meters (usually can measure into the 100 Megaohm range). However, they don't do it with the 1 to 9 volt range that you typically see in a multimeter. Instead, they use anything from a few hundred volts up to a few thousand volts. The voltage that you use should be very close to the actual nominal circuit voltage. Otherwise, if you applied say a 5,000 volt megger to a 250 volt THHN wire, the much higher voltage can make pin-hole size failures in the insulation if/when it arcs through it. This actually causes damage. Again, why megger? Well, almost any insulator can become a semiconductor under the right conditions, even carbon. If for instance a cable becomes saturated with oil, the hydrocarbons can form a pretty darned good semiconductor. And since you already have a metal in intimate contact with the jacket, the resulting electrical device can potentially be a Schottky barrier...at very low voltages and in one direction, it won't conduct much if any electricity. But the reverse bias breakdown voltage is often only a few hundred volts at most. Even without forming a true semiconductor, you can still accidentally form a MOV, a metal-oxide varistor that goes from almost a true insulator into almost a true conductor with relatively low voltages...the same device that you find in surge protectors. All these potentially nasty issues mean that in the real world of construction debris, miles of conduit, oils, greases, and who knows what else, "resistance" is a variable that is voltage dependent. This means that the relatively low voltage of a multimeter is completely insufficient for power circuits with voltages in the 100+ volt range. And you can't just use an arbitrarily high test voltage because as I stated earlier, you can actually cause an insulation fault which is what you are intending on measuring. The only way to test if there is sufficient insulation in the wiring is by using a comparable test voltage to the nominal voltages that will be passing across your wiring. Hence...the purpose of using a megger instead of a standard multimeter with a high range resistance measurement function. There are arguments in the other direction. Bender (www.bender.org) sells continuous online and offline insulation monitors that use low voltage signals, and they have been making counter-arguments to the "high voltage" camp for years. Just ask and you'll get an ear full. The related hi-pot test has also come under fire for cables that have been in use (not brand new cables) because it can actually CAUSE failures, according to research conducted by the EPRI. See: http://ecmweb.com/ops/electric_putting_hip...asture/?smte=wr OK, now that we've covered Meggers 101, let's get back to your issue...what should be the appropriate test voltage and/or specs for a thermocouple cable? Let's start with what the insulation has to achieve. Insulation on power conductors must be thick enough to prevent voltage from appearing on the surface of the jacket of the cable. This is for safety reasons...preventing it from bleeding into other cables or into a metal object in contact with the insulation, such as a conduit. If the insulation is far too thin, as described above, an arc will form and it will rapidly fail. But even above that level, it would not be good to energize a conduit that someone might touch with their bare hands from voltage bleeding out of the power conductor. NETA has determined standards for insulation ratings (impedance) for power conductors. You can reference their specification but the basic rule of thumb is that you need 1 megaohm for every kilovolt of voltage, with a bare minimum of 1 megaohm. This is as per the _American Electrician's Handbook_. They reference an IEEE standard (I forget which one) but the 1 megaohm minimum is as per the IEEE standard. NETA is the recognized standard for equipment testing for installation purposes, including hi-pot and megger testing. Any time that I write a specification for say a substation, I reference NETA standards as the testing requirement. Any company that comes out and does testing like this is very familiar with and uses NETA testing (and might even be a card carrying member). I would probably defer to a NETA standard before I cared what the wire manufacturer said because NETA is for installations, not for factory production tests. A thermocouple generates a voltage of less than 100 millivolts. Thermocouple wiring only has to have enough insulation to insulate that millivolt signal. So a conventional multimeter with the usual 1-9 volt test voltage and a scale that reads into the megaohms is more than sufficient for the job. A megger is not only overkill but has the potential to damage the cable through insulation breakdown and should NOT BE USED to test the cable. So to summarize everything above, just use a conventional multimeter that can measure resistance up to at least 1 megaohm and check for that reading, right? Not so fast. There's a very good reason that an even higher resistance is needed with this particular signal conductor. NETA doesn't really apply here because their specs are mostly for power cables. The amount of power being generated here is in the micro-ampere to nano-ampere range. Instead, the instrumentation requirements should be considered. Consider the negative effects of a low insulation resistance on measurement accuracy. Imagine for a minute how the thermocouple reader actually works. It's essentially a voltmeter. It has some impedance. The impedance is fairly high because otherwise the relatively weak thermopile would not generate enough electricity and there would be a voltage drop, screwing up the readings. In addition, the insulation of the cable appears as an impedance that is in parallel with the thermocouple reader's impedance because the conduit surrounding the cable is bonded (and grounded to Earth through a low impedance path). If the impedance of the cable insulation relative to the thermocouple reader is close to the meter's impedance, then we get a voltage reading of V_actual * (R_Cable)/(R_Meter+R_Cable). If the cable impedance is roughly equal to the meter impedance, the voltage reading will be half and we'll get a big error. At 10 times higher, the voltage loss is 9%. Considering that thermocouple accuracy is usually only 2-3%, then a voltage drop of less than that should be sufficient. 2% is a ratio of about 50:1. I looked on the data sheets for Allen Bradley equipment but found nothing. According to the manual for an Acromag thermocouple reader card (first one I could find that had a spec on this), it reads up to +/-1 VDC maximum at 25 nA of current draw, which works out to 400 kilohms, so I'd want to see an insulation resistance of at least .4 x 50 = 20 megaohms to keep the errors from insulation bleeding signal to under 2%. I don't know what other manufacturers have for an internal device impedance but since 400 kilohms is pretty close to what you get with a JFET-type voltage follower front end circuit (compared to 10-20 kilohms for a BJT-type), they are all probably pretty similar. I would be very surprised to find a follower front-end that exceeded 1 megaohm, and I would be similarly surprised to find one that was less than 100 kilohms. Edited 30 May 2009 by paulengr

[Rodney 21]

The E.M.F generated by a J type thermocouple at 0 degrees C is 0 microvolts and at 500 degees C is 27393 microvolts. As Paulengr has said the thermocouple reader is essentially a volt meter and I would say a very good one. Rodney

[Bob O 8]

Thanks to all that have replied. I am off to the customers this week and I'll let you know what they say. A week with no cell coverage or internet = middle of BFE.

[Ken Moore 23]

Hey Bob, Are you an ex-Navy guy. Haven't heard "BFE" since I got out. Ken

[TimWilborne 108]

Are you serious? BFE is right down the road around here...everyone's heard of it...unless I'm thinking of a different BFE

[Ken Moore 23]

B= vagrant F= copulation E= home of the pharaohs

[TimWilborne 108]

Yep...same place

[Bob O 8]

Hey Ken, Ex-Navy here I was an ASE stationed in Jax FL and at Cecil Field FL.