Re: [PSES] Low current Transformer OC Protection

2016-10-31 Thread Brian Ceresney 
Hello Experts,
In a previous life I spent quite a bit of time testing power-limited small 
transformers to North American requirements. One of the design strategies I 
encountered was the use of a special nylon coated wire(NEW), which would open 
safely and repeat-ably under fault conditions at a very specific temperature. 
The winding did not fail as result of current, but of elevated temperature.
This process only took a few minutes under fault conditions, and samples 
invariably passed dielectric strength tests afterwards.
Memory fails me as to the actual temperature measured in real-time, but I’d 
guess that the overall winding temperature would be 120 degrees or so when the 
winding became a fuse.
This may be an option for your application, as the transformer fails safe, but 
has a high enough impedance that inrush is not an issue.
I hope this helps.
Brian IV

Best Regards,
Brian Ceresney
Regulatory Lead,
Delta-Q Technologies Corp.
Phone 1.604.566.8827

Confidentiality Notice: This email message, including any attachments, is for 
the sole use of the intended recipient(s) and may contain confidential and 
privileged information. Any unauthorized review, use, disclosure or 
distribution is prohibited. If you are not the intended recipient, please 
contact the sender by reply e-mail and destroy all copies of the original 
message.

From: Richard Nute [mailto:ri...@ieee.org]
Sent: October-31-16 11:22 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Low current Transformer OC Protection



Hi Brian:

It sounds to me as though you have an “impedance-protected” transformer.  A 
common example in the USA is a doorbell transformer.  The output can be shorted 
all day, and the transformer will not overheat (exceed the insulation 
temperature ratings).  The transformer cannot draw or supply enough current to 
operate a fuse.  And, it cannot get hot enough (above normal temperatures) to 
operate thermal protector.

You can explain to the inspector that it is an impedance-protected transformer. 
 He may not understand, so you will have to insist that he take your 
explanation back to his office for further consideration.

Or, put a fuse in, knowing that its only purpose is to satisfy the inspector.

Or, as Brian has suggested, use a certified transformer.

Rich



-


This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
<emc-p...@ieee.org<mailto:emc-p...@ieee.org>>

All emc-pstc postings are archived and searchable on the web at: 
http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.

Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to 
unsubscribe)<http://www.ieee-pses.org/list.html>
List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:
Scott Douglas <sdoug...@ieee.org<mailto:sdoug...@ieee.org>>
Mike Cantwell <mcantw...@ieee.org<mailto:mcantw...@ieee.org>>

For policy questions, send mail to:
Jim Bacher <j.bac...@ieee.org<mailto:j.bac...@ieee.org>>
David Heald <dhe...@gmail.com<mailto:dhe...@gmail.com>>

-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
<emc-p...@ieee.org>

All emc-pstc postings are archived and searchable on the web at:
http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.

Website:  http://www.ieee-pses.org/
Instructions:  http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:
Scott Douglas <sdoug...@ieee.org>
Mike Cantwell <mcantw...@ieee.org>

For policy questions, send mail to:
Jim Bacher:  <j.bac...@ieee.org>
David Heald: <dhe...@gmail.com>

Re: [PSES] Low current Transformer OC Protection

2016-10-31 Thread Brian O'Connell 
Transformers that "don't need a fuse" need an 'inherently limited' assessment 
per UL5085-3/CSA No.66.3 and/or IEC61558-2-6.  

Limits based on load current are correct where NEC article 725 is scoped, as it 
becomes a wiring issue. For North America, the limits would be defined in CSA 
No.223 and UL1310.

DCR alone does not necessarily indicate the peak inrush for all transformers.

Brian

From: John Woodgate [mailto:jmw1...@btinternet.com] 
Sent: Monday, October 31, 2016 9:54 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Low current Transformer OC Protection

This is why small transformers that don't need a fuse were developed. They have 
a special type of coated wire for the primary winding, which goes quietly 
open-circuit if it gets too hot. However, I suppose your product standard might 
not allow them. 

The limits based on load current are technically  indefensible, but that may 
not help. The right way to determine what fuse you want is to take account of 
the worst-case inrush current, which occurs when the core is left fully 
magnetized at switch-off and the supply is switched on at the point where it is 
trying to magnetize it further. The current is then limited only by the DC 
resistance of the primary winding, and you need to look at the fuse I^2T curves 
to select one (usually it needs a T-type) that doesn't fail on inrush but does 
fail on about 1.5 times the full-load current of the transformer.  

That is most unlikely to be the 1.1 mA you mention (32 mA being 3000% of it).  
I doubt you can get a transformer rated at less than about 1.5 VA, which with 
120 V input means a full-load primary current of  12.5 mA. I measured one like 
that and the primary resistance is 600 ohms, so the inrush current could be 200 
mA. That means that you need a fuse that will pass 200 mA for at least half a 
cycle of 60 Hz, while breaking within an acceptable time at, say,  2 x 12.5 mA. 
It may indeed be difficult to find one, but at least you know what the 
component is actually required to do.

I suspect that you can find a fuse that works for a 3 VA transformer; 1.5 VA is 
very extreme.

With best wishes DESIGN IT IN! OOO – Own Opinions Only
www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

Sylvae in aeternum manent.

From: Kunde, Brian [mailto:brian_ku...@lecotc.com] 
Sent: Monday, October 31, 2016 3:59 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Low current Transformer OC Protection

Greeting Experts.

I often run into issues with safety inspectors during Field Evaluations 
regarding overcurrent protection of small transformers that are not thermally 
protected.  

These small transformers can draw such small amounts of current on both the 
primary and secondary, that finding a fuse within 300% (250%) of the primary or 
167% of the secondary max load current is impossible.  

Even if we use the smallest fuse we can find (Littelfuse 218 series is 0.032A), 
this value can be over 3000% of the load current.

Even if we provide thermal test data to an inspector, they will reply that our 
data is worthless to them as they only accept data from their own lab or other 
NRTLs.  

So am I missing something here?  The NEC says I can use the next highest common 
fuse value. Is this acceptable even if the value is exceeds 1000% of the load 
current?

Has anyone ran across this issue with inspectors?  How best do we protect small 
transformers and meet the electric codes?  

Thanks in advance.
The Other Brian




LECO Corporation Notice: This communication may contain confidential 
information intended for the named recipient(s) only. If you received this by 
mistake, please destroy it and notify us of the error. Thank you. 
-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
<emc-p...@ieee.org>
All emc-pstc postings are archived and searchable on the web at: 
http://www.ieee-pses.org/emc-pstc.html
Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.
Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html 
For help, send mail to the list administrators:
Scott Douglas <sdoug...@ieee.org>
Mike Cantwell <mcantw...@ieee.org> 
For policy questions, send mail to:
Jim Bacher <j.bac...@ieee.org>
David Heald <dhe...@gmail.com> 
-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
<emc-p...@ieee.org>
All emc-pstc postings are archived and searchable on the w

Re: [PSES] Low current Transformer OC Protection

2016-10-31 Thread Richard Nute 
 

 

Hi Brian:

 

It sounds to me as though you have an “impedance-protected” transformer.  A 
common example in the USA is a doorbell transformer.  The output can be shorted 
all day, and the transformer will not overheat (exceed the insulation 
temperature ratings).  The transformer cannot draw or supply enough current to 
operate a fuse.  And, it cannot get hot enough (above normal temperatures) to 
operate thermal protector. 

 

You can explain to the inspector that it is an impedance-protected transformer. 
 He may not understand, so you will have to insist that he take your 
explanation back to his office for further consideration.

 

Or, put a fuse in, knowing that its only purpose is to satisfy the inspector.

 

Or, as Brian has suggested, use a certified transformer. 

 

Rich

 

 

 


-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 


All emc-pstc postings are archived and searchable on the web at:
http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.

Website:  http://www.ieee-pses.org/
Instructions:  http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:
Scott Douglas 
Mike Cantwell 

For policy questions, send mail to:
Jim Bacher:  
David Heald:

Re: [PSES] Low current Transformer OC Protection

2016-10-31 Thread John Woodgate 
Limits based on load current may well be what the NEC requires, but that 
doesn't make it technically correct. 

When is Ohm's Law suspended so that the worst-case inrush current isn't 
volts/ohms?

With best wishes DESIGN IT IN! OOO – Own Opinions Only
www.jmwa.demon.co.uk J M Woodgate and Associates Rayleigh England

Sylvae in aeternum manent.


-Original Message-
From: Brian O'Connell [mailto:oconne...@tamuracorp.com] 
Sent: Monday, October 31, 2016 5:31 PM
To: John Woodgate <jmw1...@btinternet.com>; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: RE: [PSES] Low current Transformer OC Protection

Transformers that "don't need a fuse" need an 'inherently limited' assessment 
per UL5085-3/CSA No.66.3 and/or IEC61558-2-6.  

Limits based on load current are correct where NEC article 725 is scoped, as it 
becomes a wiring issue. For North America, the limits would be defined in CSA 
No.223 and UL1310.

DCR alone does not necessarily indicate the peak inrush for all transformers.

Brian

From: John Woodgate [mailto:jmw1...@btinternet.com]
Sent: Monday, October 31, 2016 9:54 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: Re: [PSES] Low current Transformer OC Protection

This is why small transformers that don't need a fuse were developed. They have 
a special type of coated wire for the primary winding, which goes quietly 
open-circuit if it gets too hot. However, I suppose your product standard might 
not allow them. 

The limits based on load current are technically  indefensible, but that may 
not help. The right way to determine what fuse you want is to take account of 
the worst-case inrush current, which occurs when the core is left fully 
magnetized at switch-off and the supply is switched on at the point where it is 
trying to magnetize it further. The current is then limited only by the DC 
resistance of the primary winding, and you need to look at the fuse I^2T curves 
to select one (usually it needs a T-type) that doesn't fail on inrush but does 
fail on about 1.5 times the full-load current of the transformer.  

That is most unlikely to be the 1.1 mA you mention (32 mA being 3000% of it).  
I doubt you can get a transformer rated at less than about 1.5 VA, which with 
120 V input means a full-load primary current of  12.5 mA. I measured one like 
that and the primary resistance is 600 ohms, so the inrush current could be 200 
mA. That means that you need a fuse that will pass 200 mA for at least half a 
cycle of 60 Hz, while breaking within an acceptable time at, say,  2 x 12.5 mA. 
It may indeed be difficult to find one, but at least you know what the 
component is actually required to do.

I suspect that you can find a fuse that works for a 3 VA transformer; 1.5 VA is 
very extreme.

With best wishes DESIGN IT IN! OOO – Own Opinions Only www.jmwa.demon.co.uk J M 
Woodgate and Associates Rayleigh England

Sylvae in aeternum manent.

From: Kunde, Brian [mailto:brian_ku...@lecotc.com]
Sent: Monday, October 31, 2016 3:59 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Low current Transformer OC Protection

Greeting Experts.

I often run into issues with safety inspectors during Field Evaluations 
regarding overcurrent protection of small transformers that are not thermally 
protected.  

These small transformers can draw such small amounts of current on both the 
primary and secondary, that finding a fuse within 300% (250%) of the primary or 
167% of the secondary max load current is impossible.  

Even if we use the smallest fuse we can find (Littelfuse 218 series is 0.032A), 
this value can be over 3000% of the load current.

Even if we provide thermal test data to an inspector, they will reply that our 
data is worthless to them as they only accept data from their own lab or other 
NRTLs.  

So am I missing something here?  The NEC says I can use the next highest common 
fuse value. Is this acceptable even if the value is exceeds 1000% of the load 
current?

Has anyone ran across this issue with inspectors?  How best do we protect small 
transformers and meet the electric codes?  

Thanks in advance.
The Other Brian




LECO Corporation Notice: This communication may contain confidential 
information intended for the named recipient(s) only. If you received this by 
mistake, please destroy it and notify us of the error. Thank you. 
-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
<emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the 
web at: http://www.ieee-pses.org/emc-pstc.html
Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.
Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how

Re: [PSES] Low current Transformer OC Protection

2016-10-31 Thread Brian O'Connell 
If the transformer is a 'certified' component, what are its ratings and 
conditions of acceptability?

Do the connected circuits in the end-use equipment have a required interrupt 
rating?

What article of the code is being applied?

Per admin law, Field assessment only done by a NRTL or AHJ. Any fuse where the 
melting integral will not cause problems with normal operations would be ok if 
the overload and s/c tests do not result in excessive winding temperatures. 
Transformers assessed per UL5085 or CSA No.66 series will indicate requirements 
for current interrupt component.

Brian


From: Kunde, Brian [mailto:brian_ku...@lecotc.com] 
Sent: Monday, October 31, 2016 8:59 AM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Low current Transformer OC Protection

Greeting Experts.

I often run into issues with safety inspectors during Field Evaluations 
regarding overcurrent protection of small transformers that are not thermally 
protected.  

These small transformers can draw such small amounts of current on both the 
primary and secondary, that finding a fuse within 300% (250%) of the primary or 
167% of the secondary max load current is impossible.  

Even if we use the smallest fuse we can find (Littelfuse 218 series is 0.032A), 
this value can be over 3000% of the load current.

Even if we provide thermal test data to an inspector, they will reply that our 
data is worthless to them as they only accept data from their own lab or other 
NRTLs.  

So am I missing something here?  The NEC says I can use the next highest common 
fuse value. Is this acceptable even if the value is exceeds 1000% of the load 
current?

Has anyone ran across this issue with inspectors?  How best do we protect small 
transformers and meet the electric codes?  

Thanks in advance.
The Other Brian




LECO Corporation Notice: This communication may contain confidential 
information intended for the named recipient(s) only. If you received this by 
mistake, please destroy it and notify us of the error. Thank you. 
-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 

All emc-pstc postings are archived and searchable on the web at: 
http://www.ieee-pses.org/emc-pstc.html
Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.
Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html 
For help, send mail to the list administrators:
Scott Douglas 
Mike Cantwell  
For policy questions, send mail to:
Jim Bacher 
David Heald  

-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 


All emc-pstc postings are archived and searchable on the web at:
http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.

Website:  http://www.ieee-pses.org/
Instructions:  http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:
Scott Douglas 
Mike Cantwell 

For policy questions, send mail to:
Jim Bacher:  
David Heald:

Re: [PSES] Low current Transformer OC Protection

2016-10-31 Thread Kunde, Brian 
John,

One transformer I’m currently looking at has the following specs:

Input = 80-300V (230V Nom) 50/60hz
Output = (@ 230V input) 2 x 5.0V @ 0.0025A
Output Power = 0.025 VA
Insulation Class “F” (155ºC).

I don’t currently have a sample transformer to measure the primary impedance.

It would be nice if the manufacturer would provide information on how to 
properly protect this device.  Any suggestions?

Thanks,
Brian


From: John Woodgate [mailto:jmw1...@btinternet.com]
Sent: Monday, October 31, 2016 12:54 PM
To: Kunde, Brian; EMC-PSTC@LISTSERV.IEEE.ORG
Subject: RE: [PSES] Low current Transformer OC Protection

This is why small transformers that don't need a fuse were developed. They have 
a special type of coated wire for the primary winding, which goes quietly 
open-circuit if it gets too hot. However, I suppose your product standard might 
not allow them.

The limits based on load current are technically  indefensible, but that may 
not help. The right way to determine what fuse you want is to take account of 
the worst-case inrush current, which occurs when the core is left fully 
magnetized at switch-off and the supply is switched on at the point where it is 
trying to magnetize it further. The current is then limited only by the DC 
resistance of the primary winding, and you need to look at the fuse I^2T curves 
to select one (usually it needs a T-type) that doesn't fail on inrush but does 
fail on about 1.5 times the full-load current of the transformer.

That is most unlikely to be the 1.1 mA you mention (32 mA being 3000% of it).  
I doubt you can get a transformer rated at less than about 1.5 VA, which with 
120 V input means a full-load primary current of  12.5 mA. I measured one like 
that and the primary resistance is 600 ohms, so the inrush current could be 200 
mA. That means that you need a fuse that will pass 200 mA for at least half a 
cycle of 60 Hz, while breaking within an acceptable time at, say,  2 x 12.5 mA. 
It may indeed be difficult to find one, but at least you know what the 
component is actually required to do.

I suspect that you can find a fuse that works for a 3 VA transformer; 1.5 VA is 
very extreme.

With best wishes DESIGN IT IN! OOO – Own Opinions Only
www.jmwa.demon.co.uk<http://www.jmwa.demon.co.uk/> J M Woodgate and Associates 
Rayleigh England

Sylvae in aeternum manent.

From: Kunde, Brian [mailto:brian_ku...@lecotc.com]
Sent: Monday, October 31, 2016 3:59 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG<mailto:EMC-PSTC@LISTSERV.IEEE.ORG>
Subject: [PSES] Low current Transformer OC Protection

Greeting Experts.

I often run into issues with safety inspectors during Field Evaluations 
regarding overcurrent protection of small transformers that are not thermally 
protected.

These small transformers can draw such small amounts of current on both the 
primary and secondary, that finding a fuse within 300% (250%) of the primary or 
167% of the secondary max load current is impossible.

Even if we use the smallest fuse we can find (Littelfuse 218 series is 0.032A), 
this value can be over 3000% of the load current.

Even if we provide thermal test data to an inspector, they will reply that our 
data is worthless to them as they only accept data from their own lab or other 
NRTLs.

So am I missing something here?  The NEC says I can use the next highest common 
fuse value. Is this acceptable even if the value is exceeds 1000% of the load 
current?

Has anyone ran across this issue with inspectors?  How best do we protect small 
transformers and meet the electric codes?

Thanks in advance.
The Other Brian




LECO Corporation Notice: This communication may contain confidential 
information intended for the named recipient(s) only. If you received this by 
mistake, please destroy it and notify us of the error. Thank you.
-


This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
<emc-p...@ieee.org<mailto:emc-p...@ieee.org>>

All emc-pstc postings are archived and searchable on the web at: 
http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.

Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to 
unsubscribe)<http://www.ieee-pses.org/list.html>
List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:
Scott Douglas <sdoug...@ieee.org<mailto:sdoug...@ieee.org>>
Mike Cantwell <mcantw...@ieee.org<mailto:mcantw...@ieee.org>>

For policy questions, send mail to:
Jim Bacher <j.bac...@ieee.org<mailto:j.bac...@ieee.org>>
David Heald <dhe...@gmail.com<mailto:dhe...@gmail.com>>

Re: [PSES] Low current Transformer OC Protection

2016-10-31 Thread John Woodgate 
This is why small transformers that don't need a fuse were developed. They have 
a special type of coated wire for the primary winding, which goes quietly 
open-circuit if it gets too hot. However, I suppose your product standard might 
not allow them. 
 
The limits based on load current are technically  indefensible, but that may 
not help. The right way to determine what fuse you want is to take account of 
the worst-case inrush current, which occurs when the core is left fully 
magnetized at switch-off and the supply is switched on at the point where it is 
trying to magnetize it further. The current is then limited only by the DC 
resistance of the primary winding, and you need to look at the fuse I^2T curves 
to select one (usually it needs a T-type) that doesn't fail on inrush but does 
fail on about 1.5 times the full-load current of the transformer.  
 
That is most unlikely to be the 1.1 mA you mention (32 mA being 3000% of it).  
I doubt you can get a transformer rated at less than about 1.5 VA, which with 
120 V input means a full-load primary current of  12.5 mA. I measured one like 
that and the primary resistance is 600 ohms, so the inrush current could be 200 
mA. That means that you need a fuse that will pass 200 mA for at least half a 
cycle of 60 Hz, while breaking within an acceptable time at, say,  2 x 12.5 mA. 
It may indeed be difficult to find one, but at least you know what the 
component is actually required to do.
 
I suspect that you can find a fuse that works for a 3 VA transformer; 1.5 VA is 
very extreme.
 
With best wishes DESIGN IT IN! OOO – Own Opinions Only
  www.jmwa.demon.co.uk J M Woodgate and 
Associates Rayleigh England
 
Sylvae in aeternum manent.
 
From: Kunde, Brian [mailto:brian_ku...@lecotc.com] 
Sent: Monday, October 31, 2016 3:59 PM
To: EMC-PSTC@LISTSERV.IEEE.ORG
Subject: [PSES] Low current Transformer OC Protection
 
Greeting Experts.
 
I often run into issues with safety inspectors during Field Evaluations 
regarding overcurrent protection of small transformers that are not thermally 
protected.  
 
These small transformers can draw such small amounts of current on both the 
primary and secondary, that finding a fuse within 300% (250%) of the primary or 
167% of the secondary max load current is impossible.  
 
Even if we use the smallest fuse we can find (Littelfuse 218 series is 0.032A), 
this value can be over 3000% of the load current.
 
Even if we provide thermal test data to an inspector, they will reply that our 
data is worthless to them as they only accept data from their own lab or other 
NRTLs.  
 
So am I missing something here?  The NEC says I can use the next highest common 
fuse value. Is this acceptable even if the value is exceeds 1000% of the load 
current?
 
Has anyone ran across this issue with inspectors?  How best do we protect small 
transformers and meet the electric codes?  
 
Thanks in advance.
The Other Brian
 
 
 
  _  


LECO Corporation Notice: This communication may contain confidential 
information intended for the named recipient(s) only. If you received this by 
mistake, please destroy it and notify us of the error. Thank you. 

-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 
 >
All emc-pstc postings are archived and searchable on the web at: 
http://www.ieee-pses.org/emc-pstc.html
Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.
Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) 
 
List rules: http://www.ieee-pses.org/listrules.html 
For help, send mail to the list administrators:
Scott Douglas  >
Mike Cantwell  > 
For policy questions, send mail to:
Jim Bacher  >
David Heald  > 

-

This message is from the IEEE Product Safety Engineering Society emc-pstc 
discussion list. To post a message to the list, send your e-mail to 


All emc-pstc postings are archived and searchable on the web at:
http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at 
http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used 
formats), large files, etc.

Website:  http://www.ieee-pses.org/
Instructions:  http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to