You are not registered yet. Please click here to register!


 
 
plc storereviewsdownloads
This board is for PLC Related Q&A ONLY. Please DON'T use it for advertising, etc.
 
Try our online PLC Simulator- FREE.  Click here now to try it.

---------->>>>>Get FREE PLC Programming Tips

New Here? Please read this important info!!!


Go Back   PLCS.net - Interactive Q & A > PLCS.net - Interactive Q & A > LIVE PLC Questions And Answers

PLC training tools sale

Reply
 
Thread Tools Display Modes
Old October 16th, 2017, 01:45 PM   #1
JeremyAdair87
Member
United States

JeremyAdair87 is offline
 
Join Date: Jun 2014
Location: Ozark Mountains
Posts: 47
Do you fuse for the Wire or Device?

I know I should go to an electrical forum for this but I am comfortable here.

I've seen two ways to fuse.

In one scenario you put a 15 amp fuse on the wire to make sure the 120v never gets hot enough to start a fire. I would use an electrical reference for the wire gauge. I am simplifying.

I've also seen say a 5 amp fuse instead because the device connected to the 120V should never pull more than 4 amps.

I got into a recent debate with another EE and I am curious which is correct. In most places that I've seen you fuse the main wires coming in for protecting the wire and then go much smaller for the devices down stream. It seems crazy to put a 15 amp fuse on a 2.3 amp controller with nothing else in parallel on the power of that wire.
  Reply With Quote
Old October 16th, 2017, 01:55 PM   #2
just the cowboy
Member
United States

just the cowboy is offline
 
just the cowboy's Avatar
 
Join Date: Nov 2013
Location: Pa
Posts: 254
Both are correct

Usually the feed to a machine or device is protected for the wire size feeding it. Once you are inside of a panel the fuse still needs to be sized to protect the wire BUT you can go down in size to protect the device. There are exceptions to the feeders for motor starting, where the fuse can be larger than the wire size as long as there is another means of checking current such as overload heaters.
__________________
Work safe, have fun, don't die
  Reply With Quote
Old October 16th, 2017, 04:00 PM   #3
LoganB
Lifetime Supporting Member
United States

LoganB is offline
 
Join Date: Apr 2017
Location: Michigan
Posts: 356
I've seen others on the forum suggest (in a debate about whether or not to ground 24v) that most of the hardware is already protected internally against surges and that fuses in the panel are all for fire protection in the wires. I offer that without my own comment or opinion, as I've just started learning the ropes (wires?) of EE.

I believe the discussion was in this thread...
http://www.plctalk.net/qanda/showthr...ghlight=ground
__________________
Present: Controls Engineer
Future: Skynet Engineer
  Reply With Quote
Old October 16th, 2017, 04:17 PM   #4
ASF
Lifetime Supporting Member
Australia

ASF is offline
 
Join Date: Jun 2012
Location: Australia
Posts: 2,316
I can't speak for 'merica, of course, but here in upside down land the general gist of it is that you need protection against two things: short circuit, and overcurrent. This applies to every component in the circuit, be that a simple piece of wire or a $10k PLC chassis.

To use the simple example of a DC power supply. I put a circuit breaker on the line side, because I need to protect the wiring to the PS and the PS itself from short circuit and overcurrent. As far as short circuit protection goes, the size of the breaker is largely irrelevant (within reason, of course). If I get a short circuit on the wiring or internally within the PS, any breaker will trip before the 240V causes any real problems, provided I haven't done something silly like put an 80A breaker on it. As far as overcurrent goes, well, I know that the PS will only ever draw, say 2A. Any more than that and it's already been damaged and no circuit protective device will save it; at that point it's just a matter of making sure it doesn't catch fire. The wiring is good up to around 10A, so any breaker from 2A to 10A would be fine. Typically, I'd use a 4A - give me a little headroom and prevent nuisance tripping, but low enough to trip on a genuine fault.

On the load side of the PS, I could argue that the PS has internal protection for both short circuit and overcurrent, and no external protection is required. I tend to put a breaker in anyway, but that's more for ease of distribution and power cycling than anything else. Quite often I'll have a 5A PS with a 6A breaker on it - the breaker is providing short circuit protection, and the PS is providing overcurrent protection. Of course, in practice, the PS will provide the short circuit protection as well - it's internal cutout works a lot quicker than a breaker can trip.
  Reply With Quote
Old October 16th, 2017, 05:09 PM   #5
rupej
Member
United States

rupej is offline
 
Join Date: Sep 2014
Location: NC
Posts: 223
^
ASF's logic is exactly how I design panels.
  Reply With Quote
Old October 16th, 2017, 06:10 PM   #6
Bit_Bucket_07
Member
United States

Bit_Bucket_07 is offline
 
Bit_Bucket_07's Avatar
 
Join Date: Feb 2013
Location: Southeast US
Posts: 1,234
The wire.
__________________
  Reply With Quote
Old October 17th, 2017, 12:32 AM   #7
janner_10
Member
United Kingdom

janner_10 is offline
 
Join Date: Dec 2014
Location: Tewkesbury
Posts: 366
Quote:
Originally Posted by ASF View Post
I can't speak for 'merica, of course, but here in upside down land the general gist of it is that you need protection against two things: short circuit, and overcurrent. This applies to every component in the circuit, be that a simple piece of wire or a $10k PLC chassis.

To use the simple example of a DC power supply. I put a circuit breaker on the line side, because I need to protect the wiring to the PS and the PS itself from short circuit and overcurrent. As far as short circuit protection goes, the size of the breaker is largely irrelevant (within reason, of course). If I get a short circuit on the wiring or internally within the PS, any breaker will trip before the 240V causes any real problems, provided I haven't done something silly like put an 80A breaker on it. As far as overcurrent goes, well, I know that the PS will only ever draw, say 2A. Any more than that and it's already been damaged and no circuit protective device will save it; at that point it's just a matter of making sure it doesn't catch fire. The wiring is good up to around 10A, so any breaker from 2A to 10A would be fine. Typically, I'd use a 4A - give me a little headroom and prevent nuisance tripping, but low enough to trip on a genuine fault.

On the load side of the PS, I could argue that the PS has internal protection for both short circuit and overcurrent, and no external protection is required. I tend to put a breaker in anyway, but that's more for ease of distribution and power cycling than anything else. Quite often I'll have a 5A PS with a 6A breaker on it - the breaker is providing short circuit protection, and the PS is providing overcurrent protection. Of course, in practice, the PS will provide the short circuit protection as well - it's internal cutout works a lot quicker than a breaker can trip.
Perfectly put.

One of the best pieces of literature I've read from an aussie!
  Reply With Quote
Old October 17th, 2017, 04:57 PM   #8
ASF
Lifetime Supporting Member
Australia

ASF is offline
 
Join Date: Jun 2012
Location: Australia
Posts: 2,316
Quote:
Originally Posted by janner_10 View Post
One of the best pieces of literature I've read from an aussie!
You obviously haven't read David Thorne
  Reply With Quote
Old October 17th, 2017, 05:47 PM   #9
daba
Lifetime Supporting Member + Moderator
United Kingdom

daba is offline
 
daba's Avatar
 
Join Date: Jul 2004
Location: uk
Posts: 4,001
"Do you fuse for the Wire or Device"

I've not read the whole thread, but IMHO you should fuse for the lowest current capability of either.

If the wire can't carry the current for the device then it is under-rated, and will inevitably be a fuse for the device.

But wires aren't designed to be fuses, they will get hot first, and the insulation may burn causing a fire.

Fuses are designed to get hot, and burn out, without the possibility of a fire.
__________________
___________________________
ControlLogix & SLC Training
a-b train ltd.
abtrain@tiscali.co.uk
www.abtrain.co.uk
tel: 07506 73 9999
nil illegitimi carborundem


  Reply With Quote
Old October 17th, 2017, 07:42 PM   #10
GaryS
Member
United States

GaryS is offline
 
GaryS's Avatar
 
Join Date: Aug 2003
Location: Lancaster Pa.
Posts: 538
Fuse for the device, insure that the wire feeding the device is adequate for the current required for the device. Branch circuit should be fused for the expected load X 1.25, the wire should be adequate for that same current level.
I like to use with, few exceptions #12 and larger for power feeds #14 and smaller for control it helps me to quickly define the use later, of course wire color helps as well.
Also you really don't use a fuse to protect the device, you use a fuse to isolate the device from the system if there is a failure in the device.

Last edited by GaryS; October 17th, 2017 at 07:44 PM.
  Reply With Quote
Old October 18th, 2017, 10:49 AM   #11
rQx
Member
Sweden

rQx is offline
 
Join Date: Oct 2010
Location: Trelleborg
Posts: 605
Step 1: You choose what fuse you need for your device
Step 2: You choose what wire size you need for that fuse
  Reply With Quote
Old October 18th, 2017, 11:21 AM   #12
janner_10
Member
United Kingdom

janner_10 is offline
 
Join Date: Dec 2014
Location: Tewkesbury
Posts: 366
Quote:
Originally Posted by ASF View Post
You obviously haven't read David Thorne
Didn't recognise the name, but when i went to the site I remembered it. It is genuinely very very funny.
  Reply With Quote
Old October 18th, 2017, 03:29 PM   #13
Toine
Member
Netherlands

Toine is offline
 
Join Date: Nov 2016
Location: NL
Posts: 119
Same here. I knew the story, yet read it again and laughed again. Thanks ASF
  Reply With Quote
Old October 18th, 2017, 04:17 PM   #14
dogleg43
Member
United States

dogleg43 is offline
 
Join Date: Dec 2005
Location: Indiana
Posts: 258
Quote:
Originally Posted by GaryS View Post
Fuse for the device, insure that the wire feeding the device is adequate for the current required for the device. Branch circuit should be fused for the expected load X 1.25, the wire should be adequate for that same current level.
I like to use with, few exceptions #12 and larger for power feeds #14 and smaller for control it helps me to quickly define the use later, of course wire color helps as well.
Also you really don't use a fuse to protect the device, you use a fuse to isolate the device from the system if there is a failure in the device.
Best explanation so far. I just couldn’t put it in words properly.
  Reply With Quote
Reply
Jump to Live PLC Question and Answer Forum

Bookmarks


Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Topics
Thread Thread Starter Forum Replies Last Post
MODBUS -> wire failure -> PLC reaction MASEngr LIVE PLC Questions And Answers 2 March 16th, 2009 11:36 PM
Analogue Inputs - Sink, Source, Help! Yabadabadoo LIVE PLC Questions And Answers 14 January 26th, 2005 12:48 PM
Turning Device and Embossing Device ferdiecobilla LIVE PLC Questions And Answers 2 December 10th, 2003 02:01 AM
replacing a high voltage fuse. saifullah99319 LIVE PLC Questions And Answers 13 July 13th, 2003 03:54 PM
Replacing ahigh voltage fuse locus LIVE PLC Questions And Answers 24 July 10th, 2003 08:41 AM


All times are GMT -5. The time now is 06:16 PM.


.