transformer fusing?

[steveng]

i have a emergency lighting ckt that feeds a .250kva xfmr 277 pri and 120 secondary, the existing circuit was installed in 77 and utilized a square d magnetic underload relay to limit the duration the the xfmr received input power, this was to allow a time interval for the hid fixtures to obtain full brightness in the event of a loss of power at night so the stairwell would be lit up. the 120v power supplies 3 incandescent fixtures over the stairwell, 2 story bldg.

these xfmrs were not fused and the underloadrelay coils are shot after 30 yrs, i am figuring on bypassing the old relays and fusing the secondary of the xfmrs, at .250 kva the load is obviously small, i counted 3 incandescent lamps over the stairwell , the connected load was approx 1 amp on the secondary
of the .250kva xfmr.

question: use a 2 amp fuse? to fuse the sec of xfmr
.250kva / 120 = 2.08 amps

the incandescent lamps are hard to change being over a stairwell and i can see why they used this timing relay to drop them out after about 3 to 5 minutes,

what other type of relay could i use to replace this underload relay?

the 277v is supplied to the relay via a lighting contactor , the relay powers the xfmr for the incandescent lamps.

thnaks
steve

 

[Smart $]

...

what other type of relay could i use to replace this underload relay?

...

Perhaps a photo cell... ???

 

[gar]

080615-0613 EST

steveng:

Use higher wattage bulbs and run at reduced voltage and lamp life can be very long. Without looking at lamp life curves I might use 80 to 90 V applied to a 120 V bulb. Intensity goes down with reduced voltage and thus the reason for a higher wattage bulb. Note: tungsten filament lamp current does not drop off as fast as the reduction in voltage. A quick measurement of a 100 W bulb resulted in a current of 78 % when the voltage was reduced to 67 %.

Why fuse the transformer secondary? Fuse the primary. In either case use a slow blow fuse. Initial inrush current of the lamp is about 10 times its steady-state value.

What is an underload relay?

Rather than being emergency lights for loss of power these appear to be supplemental light for the startup time of the HID fixtures. Is that correct or is there an emergency power source supplying these lamps?

.

 

[brian john]

What is an underload relay

Current relay?

http://www.grainger.com/Grainger/items/6C054

 

[steveng]

Current relay?

http://www.grainger.com/Grainger/items/6C054

brianjohn, how do these work?

my voltage is 277, do these come in 277?

do they work on the principle of current/time delay?

thanks

 

[steveng]

080615-0613 EST

steveng:

Use higher wattage bulbs and run at reduced voltage and lamp life can be very long. Without looking at lamp life curves I might use 80 to 90 V applied to a 120 V bulb. Intensity goes down with reduced voltage and thus the reason for a higher wattage bulb. Note: tungsten filament lamp current does not drop off as fast as the reduction in voltage. A quick measurement of a 100 W bulb resulted in a current of 78 % when the voltage was reduced to 67 %.

Why fuse the transformer secondary? Fuse the primary. In either case use a slow blow fuse. Initial inrush current of the lamp is about 10 times its steady-state value.

What is an underload relay?

Rather than being emergency lights for loss of power these appear to be supplemental light for the startup time of the HID fixtures. Is that correct or is there an emergency power source supplying these lamps?

.

thanks gar,

the incandescent lamps are on the e ckt contactor, the hid lamps are photo controlled via lighting contactor normal power, however both contactors are energized by photo cell.

the current relay is in series with the xfmr primary 277 feeding the .250 xfmr
secondary 120vac supplying incandescent lamps.

thanks for the input

 

[brian john]

They are current adjustable, and time delay adjustable. These sense current either directly from the conductor or through the use of a Current Transformer. Can operate on device being sense on or off.

 

[gar]

080615-1953 EST

steveng:

I seem to understand the following structure to your system:

There are two AC power sources, the voltage is really unimportant. One of these in the normal power company supply. The other is from some sort of emergency power supply. Maybe this is a generator or a battery supplied inverter. Again unimportant.

Loss of the lights supplied from the normal power company supply is to turn on the emergency lights. The fact that a current relay is used to detect the loss of the normal lights implies that the emergency power supply is always on and available. Thus, if even if the normal lights went off, maybe a fuse blew, then even though power company power was not lost the emergency lights would come on. That would be a good design. You would want a current sensing relay that under power loss conditions is going to close the circuit to the emergency lights. An electro-mechanical current sensing relay and using the normally closed contact would be ideal for this from a reliability point of view. An electronic one should also be available with this fail safe capability.

The above referenced current relay by brian john can probably be configured to be fail safe to a normally closed contact.

On the subject of fusing I see no need to protect the lamps (that is the output of the transformer). Rather by fusing the input to the transformer you protect the the transformer and the wires from the transformer. In all probability the current rating of the wires from the transformer to the lights is much greater than that of the transformer. 250 VA at 120 V is about 2 A and your wires are probably at least #14. A suitable slow blow fuse at the transformer input needs to tolerate the total inrush current that might be as high as 20 to 30 times the nominal lamp load you specified (I believe that was 1 A). This means only about 120 W total load.

First assume a 1 to 1 transformer with 120 V output. Lamp inrush might be 10 A. A 250 VA trasformer might be 20 A. Assume addition and we get 30 A worst case inrush. If the transformer is 277 to 120, then this translates to the primary as (120/277)*30 = 13 A peak inrush. You would like to fuse the primary at 250/277 = 0.9 A or a 1 A fuse. This would protect the transformer and be 1/(120/277) *1 = 2.3 times larger than the reflected lamp load. Can you get a slow blow fuse that will take a 13 times inrush current I do not know.

You could go larger on the fuse and provide less transformer protection but still easily blow on a secondary short circuit. This would be better than the current application where there is apparently no primary protection.

.

 

[steveng]

080615-1953 EST

steveng:

I seem to understand the following structure to your system:

There are two AC power sources, the voltage is really unimportant. One of these in the normal power company supply. The other is from some sort of emergency power supply. Maybe this is a generator or a battery supplied inverter. Again unimportant.

Loss of the lights supplied from the normal power company supply is to turn on the emergency lights. The fact that a current relay is used to detect the loss of the normal lights implies that the emergency power supply is always on and available. Thus, if even if the normal lights went off, maybe a fuse blew, then even though power company power was not lost the emergency lights would come on. That would be a good design. You would want a current sensing relay that under power loss conditions is going to close the circuit to the emergency lights. An electro-mechanical current sensing relay and using the normally closed contact would be ideal for this from a reliability point of view. An electronic one should also be available with this fail safe capability.

The above referenced current relay by brian john can probably be configured to be fail safe to a normally closed contact.

On the subject of fusing I see no need to protect the lamps (that is the output of the transformer). Rather by fusing the input to the transformer you protect the the transformer and the wires from the transformer. In all probability the current rating of the wires from the transformer to the lights is much greater than that of the transformer. 250 VA at 120 V is about 2 A and your wires are probably at least #14. A suitable slow blow fuse at the transformer input needs to tolerate the total inrush current that might be as high as 20 to 30 times the nominal lamp load you specified (I believe that was 1 A). This means only about 120 W total load.

First assume a 1 to 1 transformer with 120 V output. Lamp inrush might be 10 A. A 250 VA trasformer might be 20 A. Assume addition and we get 30 A worst case inrush. If the transformer is 277 to 120, then this translates to the primary as (120/277)*30 = 13 A peak inrush. You would like to fuse the primary at 250/277 = 0.9 A or a 1 A fuse. This would protect the transformer and be 1/(120/277) *1 = 2.3 times larger than the reflected lamp load. Can you get a slow blow fuse that will take a 13 times inrush current I do not know.

You could go larger on the fuse and provide less transformer protection but still easily blow on a secondary short circuit. This would be better than the current application where there is apparently no primary protection.

.

Gar, thanks
you're a walking electrical encylopedia, thanks for the explanation,
i could learn a lot, just reading your posts,

 

[steveng]

They are current adjustable, and time delay adjustable. These sense current either directly from the conductor or through the use of a Current Transformer. Can operate on device being sense on or off.

brianjohn, thanks,

will the one that you posted in the pic work?

i noticed it said 24vac control in the pic.

the existing current relay now in use is 277 coil voltage.