Selection on Fuse

[sye15929]

what would be the right fuse election for an electrical meter with the following parameters-

power supply input - 230 vac
power loss <2VA
overload: continuous overload -1.2 times of meter rated value, instant overload -instant current 10times/second, maximum voltage-600V (p-p)
relay output:250VAC/5A
rated voltage/current input: 220/380V, 5A

not sure how is the calculation done. pls help. would the sizing be done based on the relay output current? doesn't sound right by itself. no other useful electrical info given.

my guess is based on 1.5 of rated current input, so 1.5X5 = 7.5A, so nearest sized fuse would be 10A? Any problem with this logic? the manufacturer of the component is saying a 2A fuse would be the right one to use (no documentation or anything). taking this as an exercise only so not taking the manufacturer's word for now.

 

[ScottyUK]

You got a link to the datasheet for this meter?


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[sye15929]

ya..
here's the link

http://pmac.com.cn/Products_show.asp?lan=zh-en&skin=2&newsid=125948036

 

[ScottyUK]

Fuse the voltage inputs at as low a value as practicable, 1A or 2A would be typically the smallest available industrial HRC fuses which likely explains the manufacturer's comment on size. Fuse the auxiliary power supply at a similar value - you're really protecting the panel wiring, not the meter, but no point in putting a larger fuse in than is necessary. The meter doesn't use much power internally.

The output relay circuit... again fuse it at as low a value as you can tolerate. I wouldn't exceed 5A in any case, but in my experience it's unusual for a meter to directly switch large loads. What is the relay switching?

I assume you're using CTs with the meter. Don't fuse the CT circuits. Bad things will happen.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[waross]

Beware of very low current fuses. I fused a circuit based on the very low maximum current. The circuit would not work. With a larger fuse the circuit worked fine. I discovered that small value fuse typically have quite high resistances. The published resistance of the fuse that I had selected was in the order of 1800 Ohms.
An input voltage of 230 V and a load of 2 VA equates to a current of about 9 mA. If you find a fuse with close to a 9 mA rating expect several thousand Ohms resistance. Go with the manufacturers rating. The fuse is to protect the panel wiring as Scotty says.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

"rated voltage/current input: 220/380V, 5A"

If you are protecting the meter, this is what you need to know. 5A fuse rated at whatever your line voltage input is, in this case you said 230VAC. Even though the power consumption of the meter is very low, they likely state 5A for the reason Waross said.

On your relay output, it is telling you that the contacts are rated 5A max, but that only means you cannot exceed that with whatever the relay contacts are driving. Fusing for that circuit would be selected for whatever the relay contacts are connected to, not to exceed 5A. But the decision to use fusing on what may be a protective function is a larger discussion. We don't know what you are using this meter for.

The "overload" issue has to do with the current measuring functions (CT inputs), not the unit consumption or fusing issues.

As was said, NEVER fuse the CT circuit.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[ScottyUK]

jraef,

It's a configurable meter designed to accept CTs and VTs with the ability to program ratios, hence the comment about not putting fuses in the CT wiring. I couldn't get the datasheet pdf download to work. Might be a problem on my PC - I just changed some of the security software and am having to teach it some settings. OTOH it might be because it's a Chinese website.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[ScottyUK]

Finally loaded. Here's the link:

http://pmac.com.cn/upload/133704266.pdf

The voltage input ranges look a bit odd: the 57.5/100V range won't accommodate the majority of VTs which tend to be 63.5/110V.


----------------------------------

If we learn from our mistakes I'm getting a great education!

 

[sye15929]

thanks all! makes sense to me now.

 

[jraef]

Yes, the data sheet had worked for me, but it doesn't have that power requirement statement ion it, must have come from the manual.

They seem to have a lot of features available, but either they have really poor translators working for them or they just copied someone else's meter and they have no real understanding of what the product is suppsed to be like (highly l;ikely give they are from China). I hate to have to dig deep to find basic useful information.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376