How to overload a linear transformer with multiple protective devices

S

sankui

#1
Hello all,

I am conducting an in-house test. Our device employs a linear transformer (insulation system: CLASS B) which has two output windings (one is 10V, the other is 18V). Each output lead has one fuse (external to the transformer, F2A for 10V and F3.15A for 18V). The primary winding has one built-in thermal cutout (125 degree). Two input fuses also can be considered as the protective devices for transformer (external to the transformer, T2A). According to the standard, it seems need to do the overload test as follows:

1) Overload the 10V secondary winding on the basis of F2A. The winding under test is loaded on 4.2A (2.1x 2A) and the other winding is loaded normally. The test duration is 30 min (fuse approved per IEC 60127). F2A shall be replaced by links of negligible impedance and all other protective devices shall be in place. If second protective device operates, the test is ended.

2) Overload the 18V secondary winding on the basis of F3.15A. The winding under test is loaded on 6.6A (2.1x 3.15A) and the other winding is loaded normally. The test duration is 30 min (60127 approved fuse). F3.15A shall be replaced by links of negligible impedance and all other protective devices shall be in place. If second protective device operates, the test is ended.

3) Overload the primary winding on the basis of T2A. The winding under test is loaded on 4.2A (2.1x 2A) which realized by loading the output winding. The test duration is 30 min (60127 approved fuse). T2A shall be replaced by links of negligible impedance and all other protective devices shall be in place. If second protective device operates, the test is ended.

4) Overload test based on the built-in thermal cutout. First step is to find out the maximum test current which can not cause the fuses and thermal cutout to operate. The test shall be last until thermal stability established.

It seems so complicated. Are the above mentioned methods correct? could I do the overload test only based on the built-in thermal cutout? I think that no winding temperature would exceed the 175 degree (limit for class B) since the built-in thermal cutout is 125 degree.

Thanks and regards.
 
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Marc

Hunkered Down for the Duration with a Mask on...
Staff member
Admin
#2
Re: How to overload a linear transformer with multiple protective devices?

My Thanks in advance if anyone here can help with this one.

Where are these tests from?
 

Ninja

Looking for Reality
Staff member
Super Moderator
#3
Re: How to overload a linear transformer with multiple protective devices?

I might be talking through my hat here, so take it as it is...I read this:

You have two input fuses (In1 = 2A ; In2 = 2A)
You have two output fuses (Out1 = 2A ; Out2 = 3.15A)
You have a thermal overload protection.

1) Overload the 10V secondary winding on the basis of F2A. The winding under test is loaded on 4.2A (2.1x 2A) and the other winding is loaded normally. The test duration is 30 min (fuse approved per IEC 60127). F2A shall be replaced by links of negligible impedance and all other protective devices shall be in place. If second protective device operates, the test is ended.
Short across fuse "Out1" and apply 4.2Amps to the "Out1" circuit. Apply the normal load to the primary winding also. If you survive for 30minutes or trip a secondary protection, you pass. (In other words: If you get a 4.2A backfeed on that secondary, will it damage upstream stuff?)

2) Overload the 18V secondary winding on the basis of F3.15A. The winding under test is loaded on 6.6A (2.1x 3.15A) and the other winding is loaded normally. The test duration is 30 min (60127 approved fuse). F3.15A shall be replaced by links of negligible impedance and all other protective devices shall be in place. If second protective device operates, the test is ended.
Short across fuse "Out2" and apply 6.6Amps to the "Out2" circuit. Apply the normal load to the primary winding also. If you survive for 30minutes or trip a secondary protection, you pass. (In other words: If you get a 6.6A backfeed on that secondary, will it damage upstream stuff?)


3) Overload the primary winding on the basis of T2A. The winding under test is loaded on 4.2A (2.1x 2A) which realized by loading the output winding. The test duration is 30 min (60127 approved fuse). T2A shall be replaced by links of negligible impedance and all other protective devices shall be in place. If second protective device operates, the test is ended.
Short across fuses "In1" and "In2" and apply a load on an output winding until you get 4.2A through the primary winding. If you survive for 30min or trip another protection, you pass. (In other words: If you get a sustained backfeed on a secondary that induces a current upstream, will it break the transformer itself?)

4) Overload test based on the built-in thermal cutout. First step is to find out the maximum test current which can not cause the fuses and thermal cutout to operate. The test shall be last until thermal stability established.
Short across all of the fuses, then increase the load on the primary and track load and the temperature. Determine how hard you can load it (measured in Amps) before you trip the thermal overload protection. (in other words: How much amperage will your device pull if all of the fuses fail to perform properly? When will the thermal overload protection kick in?)

Would love another read on this from someone else...
 

Peter Selvey

Staff member
Super Moderator
#4
Re: How to overload a linear transformer with multiple protective devices?

Hello Sankui again (sorry for late post: holidays)

First point is that as a minimum each secondary must be tested separately. So for your transformer there will always be 2 tests.

Secondly, in standards there is always the principle that you only need to test protection that is relied on for safety. So, if the test based one protection (e.g. secondary fuse) proves the construction complies, other protections (e.g. primary fuse / thermal cut-out) are not safety relevant and not required to be tested (at least with respect to transformer overload).

Normally, tests based on IEC secondary fuses are quickest (only 30 minutes) and produce lower temperatures. This is because (a) primary fuses need to be higher to handle transformer in-rush and magnetising currents, and (b) the primary fuse has to handle the load from both secondaries.

If you are unsure, you can calculate the "I-squared-R" power for each condition based on the winding's dc resistance and expected test currents. It should show tests based on secondary fuses have much lower power (and hence temperatures).

The test based on the thermal cut-out (TCO) is best avoided as it takes a long time and if the TCO is single shot type, you will need two transformers. However, if during the fuse based tests the TCO opens, the test is invalid and you will need to start again with TCO based test (i.e. slow overload). So, keep an eye on the temperatures and abort the fuse test if you think the TCO will open.
 
S

sankui

#5
Re: How to overload a linear transformer with multiple protective devices?

Thanks for all the input.

Peter, your opinion is always helpful and highly appreciated. I believe that you've had a great holiday.

The only thing also confused me is that if during the fuse based tests the thermal cut-out (TCO) opens, the test is invalid and need to start again with TCO based test. Could you tell me why?

In my opinion, during the overload test which based on one protector, if another protector (in my case, the TCO) operates, the test can be considered as pass. There is also a requirement in 60601-1 (3rd)--> 15.5.1.3 overload test:
This portion of the overload test is concluded at the specified time or when a second protective device opens.
Looking forward to your reply. Thanks and regards.
 

Peter Selvey

Staff member
Super Moderator
#6
Re: How to overload a linear transformer with multiple protective devices?

My thinking is is based on the 2nd edition: if a TCO operates during fuse based test, the result is not a valid fuse test, since the 30 minutes could not be completed, nor a valid slow overload test since thermal stability was not achieved. Thus the test would need to be repeated under the 2nd edition.

This section in the 3rd edition has been significantly modified from 2nd edition, and appears to be an attempt to make it simpler but effectively the same. I would be 99% sure the committee did not intend to make any significant technical change for this point, and the quoted text will probably be cleaned up in the future to make it clearer on this point. Keep in mind the quoted text applied to all the tests in part (b), not just the overload test (based on the indent of the text).

Tests based on thermal stability give greatly different results as heat can have time to transfer to other parts including adjacent windings, terminals, and possibly double insulation (wiring, enclosure) that may be near the transformer.
 
S

sankui

#7
Re: How to overload a linear transformer with multiple protective devices?

I agree with your said. However, i can not find any sign that the quoted text will be modified in draft A1 of 60601-1:2005.

Actually, i think it is enough to prove the safety if the TCO opens in the fuse based test and thus end the overload test. Because the maximum temperature of primary winding is got (That is that temperature cause the TCO to open), no higher temperature for other windings or components nearby the transformer is expected. Of course, we need to know the rated temperature of relevant components (e.g. bobbin, enclosure...) and then compare with the obtained primary winding. Just like the exception of ball pressure test (for ball pressure test, the standard allows to check the material data only and the test is not necessary).

Further comment is highly appreciated.
 
Last edited by a moderator:

Peter Selvey

Staff member
Super Moderator
#8
Re: How to overload a linear transformer with multiple protective devices?

The overload test itself is based on a range of worst case assumptions which never happen in the real world, so the following may be considered academic ... but anyway:

Just to confirm we are talking about a thermal device that operates during a 30 minute fuse based overload test, and how this compares to a thermal device that operates during a slow overload test (which typically takes 4-8hrs). I'll call these "fast test" and "slow test".

In the case of the fast test, the temperature of the winding with the thermal device will likely be briefly hotter that would occur during a slow test. This is due to the thermal response time of the thermal device itself. But in terms of insulation damage, both time and temperature are a factor, and brief high temperates (e.g. 200C for <30min) can be less severe than long moderate temperatures (e.g. 150C for several hours). So, getting a higher winding temperature in the fast test is not a justification.

Next, for all other parts (other windings, transformer core, parts adjacent to the tranformer, enclosure), temperatures recorded in the slow test will often be much higher than the fast test. In the slow test there will be much higher energy (power x time) and time for this energy to move to other parts and reach stable temperatures. For example, a plastic enclosure near a transformer may hardly register a temperature rise during the fast test, but may easily reach melting temperatures during a slow test. Such results have been observed in practice.

So, taking these two points together, the worst case is the slow overload test, and I believe this is what the standard is asking for.
 
S

sankui

#9
Re: How to overload a linear transformer with multiple protective devices?

Dear Peter,

Thank you very much for your in-depth analysis.

From your personal info, I know that you live in Japan. Japanese people are suffering a great disaster. I really hope that the shock ends as soon as possible.

Best regards.
 

Roland chung

Trusted Information Resource
#10
Re: How to overload a linear transformer with multiple protective devices?

I am in the similar situation.

From the clause 15.5.1.3 of 3rd edition, it seems to me that the first step is to find out the FIRST operation protective device by testing (i.e. step a)) or reviewing data (in this case, perform step b) directly), then step b) is performed. The last sentence of step b) says, the overload test is concluded at the specified time or when a second protective device opens.

In my case, the transformer has two outputs. One output is 24V/ 1.0A, the other is 11V/ 0.2A. The protections are
- a primary fuse (T315mA)
- a built-in thermal link (115C)
- a secondary fuse for 11V output (T630mA)
- a secondary fuse for 24V output (T1.25A)

After reviewing the data of protectors, for both outputs, the secondary fuses will be the first operation protective device. This implies other protectors are not safety-relevant and the overload tests are just based on the secondary fuses.

Is this understanding correct?
 
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