120v control transformer on 3 Phase motor circuit

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kwired said:
with a limitation of .005 amps

50VA @ 120V = .416A - too high of current level at that voltage to be a class 2 source.
Click to expand...

I don't buy the 5 mA as being correct
especially when a 24 vac device is limited to 8 A (rated 100 VA)
24 x 8 = 192 VA let thru
Rated I is 100/24 4.2 A

a 120 vac 5 mA at <1/2 W device is useless

the limitation is for incendiary energy, not personnel safety/shock hazard

I sent the mfgr an email
every one of their numerous 120 sec class 2 xfmrs are mismarked
 
Ingenieur said:
I don't buy the 5 mA as being correct
especially when a 24 vac device is limited to 8 A (rated 100 VA)
24 x 8 = 192 VA let thru
Rated I is 100/24 4.2 A

a 120 vac 5 mA at <1/2 W device is useless

the limitation is for incendiary energy, not personnel safety/shock hazard

I sent the mfgr an email
every one of their numerous 120 sec class 2 xfmrs are mismarked
Click to expand...
The class 2 source requirements have to do with electrocution and fire hazard producing abilities.

Put your fingers on the 24VAC terminals and then put them on the 120 volt terminals and tell me which one you feel something with. Quite frankly I am a little surprised 120 volts is allowed to be class 2 at all, but it at least is limited to 5 mA.
 
kwired said:
The class 2 source requirements have to do with electrocution and fire hazard producing abilities.

Put your fingers on the 24VAC terminals and then put them on the 120 volt terminals and tell me which one you feel something with. Quite frankly I am a little surprised 120 volts is allowed to be class 2 at all, but it at least is limited to 5 mA.
Click to expand...

the 5 mA is across a short and a load, if the chart is correct
if v = 120 and i is clamped at 0.005 A the load is 24k ohm
not sure what the shock i threshhold of perception is
let go is 20-30 mA

makes no sense
at 30 vac and an 8 A limit that is 240 VA
for 120 x 0.005 = 0.60 VA
That is a factor of 400

so both 50 VA
30 V let thru is 8 A
120 is 0.005 A

V power ratio is 5^2 or 25
I is 1/(8/0.005)^2 or 4 x 10^-7
product is 100 x 10^-7
120 is 10^5 times more dangerous than 30?

illogical
energy delivered is I^2 x time
for the same time the 30 at 8 A delivers more than 120 at 1.2 (assuming a let thru 3 times rated)
8^2 / 1.2^2 = 44 times more
nope
 
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I expect that kwired has it correct...it has to do with the shock hazard.
Class 2 Circuit. The portion of the wiring system between the load side of a Class 2 power source and the connected equipment.
Due to its power limitations, a Class 2 circuit considers safety from a fire initiation standpoint and provides acceptable protection from electric shock
Click to expand...

Given thatthe trip point of a GFCI is 5 mA, a 5 mA limit on the output current of a 30 to 150 volt Class 2 power supply would provide acceptable protection from electric shock.
 
Ingenieur said:
the 5 mA is across a short and a load, if the chart is correct
if v = 120 and i is clamped at 0.005 A the load is 24k ohm
not sure what the shock i threshhold of perception is
let go is 20-30 mA

makes no sense
at 30 vac and an 8 A limit that is 240 VA
for 120 x 0.005 = 0.60 VA
That is a factor of 400

so both 50 VA
30 V let thru is 8 A
120 is 0.005 A

V power ratio is 5^2 or 25
I is 1/(8/0.005)^2 or 4 x 10^-7
product is 100 x 10^-7
120 is 10^5 times more dangerous than 30?

illogical
energy delivered is I^2 x time
for the same time the 30 at 8 A delivers more than 120 at 1.2 (assuming a let thru 3 times rated)
8^2 / 1.2^2 = 44 times more
nope
Click to expand...

But human skin resistance is relatively high and if only contacting a source of 24 volts the current will be low enough to not not even be felt in most instances. Probably well below the 5 mA that 120 volts may be able to push through a person
 
Ingenieur said:
the 5 mA is across a short and a load, if the chart is correct
if v = 120 and i is clamped at 0.005 A the load is 24k ohm
not sure what the shock i threshhold of perception is
let go is 20-30 mA
Click to expand...
The let go for the lower 5 percentile of adult females is less than 10 mA. Children have an even lower let go threshold.

makes no sense
at 30 vac and an 8 A limit that is 240 VA
for 120 x 0.005 = 0.60 VA
That is a factor of 400
Click to expand...
Many humans cannot even feel a 30 volt source.

so both 50 VA
30 V let thru is 8 A
120 is 0.005 A
Click to expand...
Many sources say that 30 volts AC, other than in water, is not a human shock hazard.

V power ratio is 5^2 or 25
I is 1/(8/0.005)^2 or 4 x 10^-7
product is 100 x 10^-7
120 is 10^5 times more dangerous than 30?
Click to expand...
No idea what you are talking about.

illogical
energy delivered is I^2 x time
for the same time the 30 at 8 A delivers more than 120 at 1.2 (assuming a let thru 3 times rated)
8^2 / 1.2^2 = 44 times more
nope
Click to expand...
So submit a public input for the 2020 code to make it logical.
 
don_resqcapt19 said:
The let go for the lower 5 percentile of adult females is less than 10 mA. Children have an even lower let go threshold.

Many humans cannot even feel a 30 volt source.

Many sources say that 30 volts AC, other than in water, is not a human shock hazard.

No idea what you are talking about.

So submit a public input for the 2020 code to make it logical.
Click to expand...

The let go varies depending on source (Dalziel, IEEE, ISO, etc)
ac or dc, frequency, body weight
for 60 hz 175 lb man 20-30 mA

body R varies
Dalziel used 500 at times
MSHA 1500
ISO 800-1200
IEEE 1000
nominal depending on path, moisture, contact area, clothing, etc

24/1000 is 24 mA and would be felt ( pain and severe shock, difficulty breathing, loss of muscle control, 99.5% of men)
and according to you clutched, not released

Dalziel mA fibrillation = 165 / sqrt time
165 ~ body wt lbs
assuming 24 mA time = 40 sec
without GF protection might killa person

I know
Factoring the squared product of voltage and current for 30v/8 A and 120v/0.005 A
the 30 has 100,000 times more energy
based on current alone the 30 v at 8 A vs 120/0.005 or (8/0.005)^2 = 2.5 million as much energy

don't really care enough
not an issue either way

not a lot of use for a power supply/xfmr
120 vac
clamped at 0.005 A
0.6 VA
0.5 W when used with a likely inductive load like a relay, lamp, small motor


ps TJ never said your sig quote
 
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Ingenieur said:
The let go varies depending on source (Dalziel, IEEE, ISO, etc)
ac or dc, frequency, body weight
for 60 hz 175 lb man 20-30 mA

body R varies
Dalziel used 500 at times
MSHA 1500
ISO 800-1200
IEEE 1000
nominal depending on path, moisture, contact area, clothing, etc

24/1000 is 24 mA and would be felt ( pain and severe shock, difficulty breathing, loss of muscle control, 99.5% of men)
and according to you clutched, not released

Dalziel mA fibrillation = 165 / sqrt time
165 ~ body wt lbs
assuming 24 mA time = 40 sec
without GF protection might killa person

I know
Factoring the squared product of voltage and current for 30v/8 A and 120v/0.005 A
the 30 has 100,000 times more energy
based on current alone the 30 v at 8 A vs 120/0.005 or (8/0.005)^2 = 2.5 million as much energy

don't really care enough
not an issue either way

not a lot of use for a power supply/xfmr
120 vac
clamped at 0.005 A
0.6 VA
0.5 W when used with a likely inductive load like a relay, lamp, small motor
Click to expand...
So really what you are saying is that the ~5mA trip for GFCIs is wrong and should be changed.
 
Ingenieur said:
The let go varies depending on source (Dalziel, IEEE, ISO, etc)
ac or dc, frequency, body weight
for 60 hz 175 lb man 20-30 mA

body R varies
Dalziel used 500 at times
MSHA 1500
ISO 800-1200
IEEE 1000
nominal depending on path, moisture, contact area, clothing, etc

24/1000 is 24 mA and would be felt ( pain and severe shock, difficulty breathing, loss of muscle control, 99.5% of men)
and according to you clutched, not released

Dalziel mA fibrillation = 165 / sqrt time
165 ~ body wt lbs
assuming 24 mA time = 40 sec
without GF protection might killa person

I know
Factoring the squared product of voltage and current for 30v/8 A and 120v/0.005 A
the 30 has 100,000 times more energy
based on current alone the 30 v at 8 A vs 120/0.005 or (8/0.005)^2 = 2.5 million as much energy

don't really care enough
not an issue either way

not a lot of use for a power supply/xfmr
120 vac
clamped at 0.005 A
0.6 VA
0.5 W when used with a likely inductive load like a relay, lamp, small motor


ps TJ never said your sig quote
Click to expand...

Yet none of that changes the fact that class 2 circuits are intended to be safe to touch.
 
Ingenieur said:
I don't buy the 5 mA as being correct
especially when a 24 vac device is limited to 8 A (rated 100 VA)
24 x 8 = 192 VA let thru
Rated I is 100/24 4.2 A

a 120 vac 5 mA at <1/2 W device is useless

the limitation is for incendiary energy, not personnel safety/shock hazard

I sent the mfgr an email
every one of their numerous 120 sec class 2 xfmrs are mismarked
Click to expand...

Please post a link to some of these transformers.
 
infinity said:
The 018 is 120 volt secondary which is not Class2. Also does one side needs to be grounded to make a grounded conductor?
Click to expand...
The logo on the sheet strongly suggests that it is a Class 2 transformer as just below the UL logo, but when I look up the actual listings on the UL site, you are correct that the TR50VA018 is not a listed Class 2 transformer.
 
don_resqcapt19 said:
The logo on the sheet strongly suggests that it is a Class 2 transformer as just below the UL logo, but when I look up the actual listings on the UL site, you are correct that the TR50VA018 is not a listed Class 2 transformer.
Click to expand...

It's probably just a boilerplate template used for their transformers which are mostly Class2. Would be the first time that the manufacturer's literature contained a mistake.

So let's get back on track and head back to the OP. Let's say that this is not Class2, what is required as far as an OCPD(s) is/are concerned?
 
infinity said:
...
So let's get back on track and head back to the OP. Let's say that this is not Class2, what is required as far as an OCPD(s) is/are concerned?
Click to expand...
As I noted back in Post #7...
Smart $ said:
...
Your 480:120 xfmr is required to have protection. At 50VA puts it under 2A on primary. Being a 2-wire secondary you can use primary only protection... but it cannot be greater than 300%, which is 50VA÷480V×300%=0.3125A.
...
Click to expand...
I'm quite tired right now, and a bit of delirium is setting in, but I believe the secondary conductor would be considered protected by the primary.
 
infinity said:
It's probably just a boilerplate template used for their transformers which are mostly Class2. Would be the first time that the manufacturer's literature contained a mistake.

So let's get back on track and head back to the OP. Let's say that this is not Class2, what is required as far as an OCPD(s) is/are concerned?
Click to expand...
Same as for any other transformer...450.3 applies. Specifically Table 450.3(B) and its notes, and as with any other transformer 240.21(C) would apply to the secondary conductors.
 
Thank you for the help here. It was an interesting conversation. I will plan on installing primary fusing.

I am thinking of an inline fuse holder. Any suggestions on where to install it? I don't have a lot of options other than the 1900 box. Is the JB an acceptable location for the fuse? How would I calculate box fill?

34ov7me.jpg
 
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