Power Supplies
- Thread starter cpal
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hbiss
EC, Westchester, New York NEC: 2014
- Location
- Hawthorne, New York NEC: 2014
- Occupation
- EC
Why would you need to know? If the nameplate says CL2 that's all you need to know.
-Hal
-Hal
It does matter!!
It does matter!!
Because I have seen Class 2 plug-in transformers go down in flames from accidental short circuits!
These were 24volt 1.2 amp for CCTV cameras.
Newer units are fuse protected and have a warning label like "Secondary fused - Do Not Short Output."
A Class 2 rating on a power source means that it's total output current is limited, but is not a promise that it can't or won't fail catastrophically if the current limit is exceeded.
It does matter!!
hbiss said:
Because I have seen Class 2 plug-in transformers go down in flames from accidental short circuits!
These were 24volt 1.2 amp for CCTV cameras.Newer units are fuse protected and have a warning label like "Secondary fused - Do Not Short Output."
A Class 2 rating on a power source means that it's total output current is limited, but is not a promise that it can't or won't fail catastrophically if the current limit is exceeded.
Thanks
And the need to know is specifically the reason for the question. I guess the mfg should be able to give an answer as to wither a power supply is or is not hinherintly protected, but that is not always the easiest path.
Tables 11(A) and 11(B)
For listing purposes, Table 11(A) and Table 11(B) provide
the required power source limitations for Class 2 and
Class 3 power sources. Table 11(A) applies for alternatingcurrent
sources, and Table 11(B) applies for direct-current
sources.
The power for Class 2 and Class 3 circuits shall be
either (1) inherently limited, requiring no overcurrent protection,
or (2) not inherently limited, requiring a combination
of power source and overcurrent protection. Power
sources designed for interconnection shall be listed for the
purpose.
This is the question, if the supply is not marked how do you know???
Charlie
And the need to know is specifically the reason for the question. I guess the mfg should be able to give an answer as to wither a power supply is or is not hinherintly protected, but that is not always the easiest path.
Tables 11(A) and 11(B)
For listing purposes, Table 11(A) and Table 11(B) provide
the required power source limitations for Class 2 and
Class 3 power sources. Table 11(A) applies for alternatingcurrent
sources, and Table 11(B) applies for direct-current
sources.
The power for Class 2 and Class 3 circuits shall be
either (1) inherently limited, requiring no overcurrent protection,
or (2) not inherently limited, requiring a combination
of power source and overcurrent protection. Power
sources designed for interconnection shall be listed for the
purpose.
This is the question, if the supply is not marked how do you know???
Charlie
macmikeman
Senior Member
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- Planet macmikeman
I was going to suggest just shorting it, but then you have to throw it away.
hbiss
EC, Westchester, New York NEC: 2014
- Location
- Hawthorne, New York NEC: 2014
- Occupation
- EC
Because I have seen Class 2 plug-in transformers go down in flames from accidental short circuits! These were 24volt 1.2 amp for CCTV cameras.
Newer units are fuse protected and have a warning label like "Secondary fused - Do Not Short Output."
A Class 2 rating on a power source means that it's total output current is limited, but is not a promise that it can't or won't fail catastrophically if the current limit is exceeded.
Quite true but this is a safety issue that you as a technician have to address. This is the exact same issue that affected fluorescent ballasts. The UL at some point in the past required internal thermal protection. If we come across an old ballast that is not thermally protected we change it out. Same here, if there is any doubt replace the transformer. It's cheap insurance.
Charlie- I've said it before, those tables are meaningless for our purposes. You have to rely on the label.
-Hal
Newer units are fuse protected and have a warning label like "Secondary fused - Do Not Short Output."
A Class 2 rating on a power source means that it's total output current is limited, but is not a promise that it can't or won't fail catastrophically if the current limit is exceeded.
Quite true but this is a safety issue that you as a technician have to address. This is the exact same issue that affected fluorescent ballasts. The UL at some point in the past required internal thermal protection. If we come across an old ballast that is not thermally protected we change it out. Same here, if there is any doubt replace the transformer. It's cheap insurance.
Charlie- I've said it before, those tables are meaningless for our purposes. You have to rely on the label.
-Hal
Absolutely. When we encountered the old style trans. they would be replaced.
Especially as in one case they were mounted in a remote location, up in an "atrium" area in a shopping mall.
We had also thrown out the unprotected ones left in our inventory.
The best kind, for both safety and serving your customers.
The f-lamp ballasts were indeed quite nasty. Not only the horrrible smell (think moldy fish meets old sneakers meets burning rubber and tar), but it was documented in a few cases that the jets of hot flame/gas ejected burned into ceramic tiles!!
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