What is meant by the statement "variation at all measured points does not exceed 1K/h ? 2

[DaSnipeKid]

I am trying to understand a temperature rise test that is being conducted over a switchboard. The test conforms to IEC 61439-1. The clause 10.10.2.3.1 (Method of test - General) regarding the temperature rise states about the constant value. It says,

"The constant value is reached, in practical condition, when the variation at all measured points (including the ambient air temperature) does not exceed 1K/h (1 kelvin per hour)"

By this statement, I can infer that, if the temperature rise at a specific joint at 15:00 is 78.5K and at 16:00 is 80K, then the total variation is 1.5K over the period of an hour. Thus this is not a constant value. At 17:00 it reaches 80.5K. Thus now, the total variation is 0.5K over the period of an hour. Thus this can be inferred as a constant value as per the clause, and we can stop the test. To simplify the scenario, refer below chronological sequence

[ol 1]
[li]15:00 - 78.5K[/li]
[li]16:00 - 80K[/li]
[li]17:00 - 80.5K[/li]
[/ol]

The question arises is, whether my inference is correct ? I am curious since, the test coordinator insisted to extend the test till 18:00 to check if the variation is below 1K/h, which, as per my opinion, against the clause in the standard. Requesting all to please clear my thoughts if I am wrong.

For reference I am attaching a snap of the sentence pertaining to the clause.

 

[zeusfaber]

I suspect the test coordinator has been caught out before and decided that avoiding having to run the whole test again was worth an extra hour.

When you're only measuring to the nearest half degree, the difference between a cold 80 K and a hot 80.5 K could be as near 1 K as makes no difference. Unless there was a really good reason for pulling the plug the very instant it looked like you might have scraped over the line, I'd have been cautious too.

A.

 

[Gr8blu]

The typical way that requirement is specified is that the object under test must remain within the allowable rise limitation (in your case, 1 K) for TWO consecutive measurement intervals (in your case, 1 hour). This means it remains stable for an extended period of time (at least 2 hours, in your case - where the temperature difference between time 1 and time 3 is within the limit). Alternative specifications can include 0.5 K/30 min ... in which case, the measurement must remain within 0.5 K for 1.5 hours.

The reason for this is that sometimes things reach a temporary thermal plateau - and then keep going up. Just because they are on the plateau does not mean thermal equilibrium is reached.

Converting energy to motion for more than half a century

 

[DaSnipeKid]

@Gr8blu

Thanks for writing. However, if I go word by word, the IEC haven't mentioned about TWO consecutive measurement interval. The one measurement interval (i.e. per hour) is sufficient to establish that the temperature is constant on the reasonable assumption that the range will only be asymptotically shrinking down to some thermal noise level.

 

[Compositepro]

Two measurements are required to remove the effects of any random measurement error. When one dries something to "constant weight", for example, two intervals of no weight change are required to confirm that the sample is,in fact, dry. This is a basic scientific procedure to insure that the measurement is correct.

 

[Gr8blu]

Actually Kid, the important part is not just meeting the criteria at ONE measurement point. It is about meeting it at ALL measurement points SIMULTANEOUSLY. For a switchgear bank, the instrumented surfaces would likely be a representative selection of the bus work in each sub-compartment of the gear, the gear enclosure surface (inside AND out), the ambient cooling medium (inside the compartment(s) and the greater room-at-large in proximity to the gear), maybe some specific joints where bus work is connected, maybe on certain discrete devices within the enclosure, etc. The chances of ALL of them reaching equilibrium (which is what the rise-per-specified-period is supposed to represent) at the same time is pretty much non-existent. On top of that, the reasonable assumption is that thermal plateaus are inevitable - it just is a matter of where they occur, and for what duration they exist.

Case 1: bearing. A bearing heats up reasonably quickly to a "stable" operating temperature when the lubrication is properly done. But the temperature reached for an improperly lubricated bearing can be vastly different - as it can for one exposed to unusual forces acting in unforeseen (or at least unexpected) directions.

Case 2: wiring within the switchgear. Passing current through a small conductor allows the conductor material to reach thermal equilibrium quickly. But what happens when the larger bus that is adjacent to it gets to temperature and raises the local ambient by several tens of degrees? Is the conductor temperature still "stable"? Or does it jump to a whole new level?

Converting energy to motion for more than half a century

 

[che12345]

@ #1. Mr DaSnipeKid (Electrical)(OP)2 May 23 14:21
@ #2. Gr8blu (Electrical)3 May 23 12:19

1. "....By .... IEC 61439-1..., if the temperature rise at a specific joint at 15:00 is 78.5K and at 16:00 is 80K, then the total variation is 1.5K over the period of an hour. Thus this is not a constant value. At 17:00 it reaches 80.5K. Thus now, the total variation is 0.5K over the period of an hour. Thus this can be inferred as a constant value as per the clause, and we can stop the test".
1.1 I fully agreed with your learned opinion. The Standard does NOT stated that the 1K/h shall be maintained for an additional one hour for conformation.
1.2 If based on ambient temperature of say 20 [sup]o[/sup] C, the joint temperature is taken as 100 [sup]o[/sup] C. NOT 100.5 [sup]o[/sup] C. See below 3.

2. "...The typical way that ....within the allowable rise limitation (in your case, 1 K) for TWO consecutive measurement intervals (in your case, 1 hour). This means it remains stable for an extended period of time (at least 2 hours, in your case - where the temperature difference between time 1 and time 3 is within the limit). Alternative specifications can include 0.5 K/30 min ... in which case, the measurement must remain within 0.5 K for 1.5 hours.
The reason for this is that sometimes things reach a temporary thermal plateau - and then keep going up. Just because they are on the plateau does not mean thermal equilibrium is reached"/color].
2. With due respect, I beg to defer from the learned advice. See above 1.

3. Caution: If based on the ambient temperature of say 20 [sup]o[/sup] C , 78.5K =98.5 [sup]o[/sup] C; 80K = 100 [sup]o[/sup] C, 80.5K= 100.5 [sup]o[/sup] C. These temperature are at about the extremes of a silver-plated copper busbar joint in the LV switchgear standard.
Che Kuan Yau (Singapore)

 

[DaSnipeKid]

@che12345 ,

Thanks for your response. The values are based on ambient of average 35[sup]0[/sup]C and peak 40[sup]0[/sup]C.
Also, I have mentioned the temperature values for ACB-copper joint. The ACB is covered by IEC 60947-2 and it states that the max. temperature rise should not exceed 80K.
As per your response, "1.1 I fully agreed with your learned opinion. The Standard does NOT stated that the 1K/h shall be maintained for an additional one hour for conformation. 1.2 If based on ambient temperature of say 20[sup]0[/sup]C, the joint temperature is taken as 100 o C. NOT 100.5[sup]0[/sup]C. See below 3."
Can I confirm that an extra hour is not needed ?

 

[che12345]

Mr DaSnipeKid (Electrical)(OP)4 May 23 09:00
".... The values are based on ambient of average 350C and peak 400C.
....#1. I have mentioned the temperature values for ACB-copper joint. The ACB is covered by IEC 60947-2 and it states that the max. temperature rise should not exceed 80K.
.....
#2 As per your response, "1. ...... The Standard does NOT stated that the 1K/h shall be maintained for an additional one hour for conformation. ......Can I confirm that an extra hour is not needed ? "

1. Per IEC 610947-2 Table 7 Temperature-rise limits for termination and accessible parts implies that NOT only terminals for external connects (80K) , the other accessories shall also comply with the temperature-rise limit stipulated.
2. Yes, an extra hour is NOT required.
2.1 I am NOT speaking for LOVAG or any accredited testing station. My humble opinion is based on I personally witnessed numerous tests on LV ACB and Switchgear and Controlgear conducted by the testing stations in England, Australia and Sweden.
Che Kuan Yau (Singapore)

 

[DaSnipeKid]

@che12345. Thank you for the response.