alawrence
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| posted on 30/1/11 at 03:17 PM |
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your swimming against the tide here mate ...just let it go
"when in a hole stop digging"
or go to college perhaps...
xx
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britishtrident
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| posted on 30/1/11 at 04:33 PM |
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quote:
Originally posted by Jasongray5
quote:
Originally posted by rusty nuts
Putting a fuse on the neg leaves a large part of the curcuit unprotected!
How? If it breaks the circuit?
Fine if only one earth path but from experience long experience this isn't the case on nearly every car I have ever worked. on.
I have seen cars with broken earth braids with the starter current earth current going through the choke cable.
[Edited on 30/1/11 by britishtrident]
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alawrence
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| posted on 30/1/11 at 04:43 PM |
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if you break the connection to the neg terminal there is no possibility of any current flow...
simples !
i don't care how long you've been working on cars chap you cant deny the laws of physics
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daviep
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| posted on 30/1/11 at 04:51 PM |
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quote:
Originally posted by rusty nuts
Some people obviously don't know thiere arse from thier elbow!
I'm not sure who this is aimed at as yourself and ash seem to be the only people who are confused as to how this works.
The OP was asking whether a fuse at the negative terminal of the battery would would prevent a fire should the main starter supply cable short circuit
on his exhaust.
The answer is yes, any current which flows from the positive also has to flow back to the negative so will blow the fuse and then whatever the cable
has shorted against will no longer be connected to negative and no circuit is possible.
For a main fuse to protect the wiring from the battery to starter this is fine, for any other circuit which has a lower current limit it would not
work fusing the negative side and these should be fused on the positive as close to the positive supply as possible.
Davie
“A truly great library contains something in it to offend everyone.”
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daviep
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| posted on 30/1/11 at 04:54 PM |
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quote:
Originally posted by britishtrident
quote:
Originally posted by Jasongray5
quote:
Originally posted by rusty nuts
Putting a fuse on the neg leaves a large part of the curcuit unprotected!
How? If it breaks the circuit?
Fine if only one earth path but from experience long experience this isn't the case on nearly every car I have ever worked. on.
I have seen cars with broken earth braids with the starter current earth current going through the choke cable.
[Edited on 30/1/11 by britishtrident]
There must still have been something connecting the negative of the battery to somewhere.
If you remove the negative terminal from the battery then you can happily touch the postive to anywhere on the car nothing will happen.
The crucial thing is that the fuse would have to be the only connection to the negative side of the battery.
Davie
“A truly great library contains something in it to offend everyone.”
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ashg
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| posted on 30/1/11 at 05:26 PM |
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electricity will always find earth if given half a chance. common practice on cars is to earth the battery to the body and the engine meaning you
would need two fuses on the negative side. if one of the fuses blew there is a risk that ground would still be found through the other connection
defeating the point of the fuse. obviously if you only have one earth off the battery that is fused you wouldn't have the problem but the large
fuse for the starter would provide no protection for the lower powered devices on the car e.g ecu, relays etc.
at the end of the day the current needed to blow a 250amp fuse is massive, by the time the fuse has blown you would already have plenty of sparks to
set a leaking fuel line on fire.
also if the pvc covering on the negative side rubs away and touches the body of the car it wont blow the fuse but it may bypass it if its after the
fuse which is useless
if you think i am being stupid ask yourself how many production cars have you seen with fuses on the negative side? im yet to recall one.
Anything With Tits or Wheels Will cost you MONEY!!
Haynes Roadster (Finished)
Exocet (Finished & Sold)
New Project (Started)
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ReMan
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| posted on 30/1/11 at 06:08 PM |
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I suggest this thread is deleted as it contains so much bad and contradictory advice 
Edited to say "potentially dangerously bad"
[Edited on 30/1/11 by ReMan]
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alawrence
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| posted on 30/1/11 at 07:11 PM |
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quote:
Originally posted by daviep
quote:
Originally posted by rusty nuts
Some people obviously don't know thiere arse from thier elbow!
I'm not sure who this is aimed at as yourself and ash seem to be the only people who are confused as to how this works.
The OP was asking whether a fuse at the negative terminal of the battery would would prevent a fire should the main starter supply cable short circuit
on his exhaust.
The answer is yes, any current which flows from the positive also has to flow back to the negative so will blow the fuse and then whatever the cable
has shorted against will no longer be connected to negative and no circuit is possible. CORRECT
For a main fuse to protect the wiring from the battery to starter this is fine, for any other circuit which has a lower current limit it would not
work fusing the negative side and these should be fused on the positive as close to the positive supply as possible.
ALSO CORRECT(however i don't recall this being brought up)
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alawrence
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| posted on 30/1/11 at 07:24 PM |
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quote:
Originally posted by britishtrident
quote:
Originally posted by Jasongray5
quote:
Originally posted by rusty nuts
Putting a fuse on the neg leaves a large part of the curcuit unprotected!
How? If it breaks the circuit?
Fine if only one earth path but from experience long experience this isn't the case on nearly every car I have ever worked. on.
I have seen cars with broken earth braids with the starter current earth current going through the choke cable.
[Edited on 30/1/11 by britishtrident]
I believe your confusing a poor earth on a device( ie an indicator , starter or wiper motor) and a broken/disconnected connection on the negative
terminal on a battery (which is what this thread was originally about)
If you have a poor earth on a device it will often find a path of lowest resistance back to the negative terminal. this is classically demonstrated
when training 1st year automotive electricians by increasing the resistance of the return path of an indicator lamp and the result is that the brake
lights will also start to flash.
However if you break the connection to the negative terminal you will have no potential difference and therefore no current flow.
Hope thats clear
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alawrence
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| posted on 30/1/11 at 07:37 PM |
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I'm not even going to try and give a detailed response to such mixed up logic except to say that i think that ReMan is correct.
suffice to say that "i refer the honorable gentleman to the answer i gave some moments before...POPPYCOCK"
quote:
Originally posted by ashg
electricity will always find earth if given half a chance. common practice on cars is to earth the battery to the body and the engine meaning you
would need two fuses on the negative side. if one of the fuses blew there is a risk that ground would still be found through the other connection
defeating the point of the fuse. obviously if you only have one earth off the battery that is fused you wouldn't have the problem but the large
fuse for the starter would provide no protection for the lower powered devices on the car e.g ecu, relays etc.
at the end of the day the current needed to blow a 250amp fuse is massive, by the time the fuse has blown you would already have plenty of sparks to
set a leaking fuel line on fire.
also if the pvc covering on the negative side rubs away and touches the body of the car it wont blow the fuse but it may bypass it if its after the
fuse which is useless
ReMan
if you think i am being stupid ask yourself how many production cars have you seen with fuses on the negative side? im yet to recall one.
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alawrence
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| posted on 30/1/11 at 07:43 PM |
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Jason
if this wasn't such a serious topic it would be quite entertaining.
i would suggest you seek advice from an electrical engineer or an auto electrician who will take some responsibility for the advise given
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daviep
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| posted on 30/1/11 at 09:08 PM |
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quote:
Originally posted by ashg
electricity will always find earth if given half a chance. common practice on cars is to earth the battery to the body and the engine meaning you
would need two fuses on the negative side. if one of the fuses blew there is a risk that ground would still be found through the other connection
defeating the point of the fuse. obviously if you only have one earth off the battery that is fused you wouldn't have the problem but the large
fuse for the starter would provide no protection for the lower powered devices on the car e.g ecu, relays etc.
at the end of the day the current needed to blow a 250amp fuse is massive, by the time the fuse has blown you would already have plenty of sparks to
set a leaking fuel line on fire.
also if the pvc covering on the negative side rubs away and touches the body of the car it wont blow the fuse but it may bypass it if its after the
fuse which is useless
if you think i am being stupid ask yourself how many production cars have you seen with fuses on the negative side? im yet to recall one.
Common practice on cars is to connect the battery to the body and then the engine to the body, giving rise to the phenomenon which BT described. This
is not the only way but it is the common way.
I did say that the fuse would have to be the ONLY connnection to the negative.
The large fuse is not protect the starter it's to prevent the wiring glowing like a heater and catching fire, if you have fuel leaks on your
vehicles I would suggest repairing them before you go too much farther.
The large fuse is not to protect anything else, anything else should be fused as close to the positive as possible.
The current to blow a 250amp fuse is indeed large, infact it's about 250amps
You are entirely correct that if the insulation rubbed through between the battery and the fuse then the fuse would be useless, however if the same
situation occured with a fuse on the positive side then the fuse would still be useless and you would have a dead short and possibly a fire.
I never said that you were being stupid but you did say "having a fuse on the negative side is as good as not having a fuse at all" which
is clearly incorrect.
If you feel that any of the advice I've given or points I've made are incorrect please feel free to correct me, every day is a school day
Cheers
Davie
“A truly great library contains something in it to offend everyone.”
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alawrence
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| posted on 30/1/11 at 09:18 PM |
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I agree
(however with a gun like that if you said BEC were best I would still agree...)
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britishtrident
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| posted on 30/1/11 at 10:07 PM |
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I love "experts" who make sweep statements condemning good sound sensible advice but miss some thing very obvious.
:-)
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daviep
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| posted on 30/1/11 at 10:49 PM |
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quote:
Originally posted by britishtrident
I love "experts" who make sweep statements condemning good sound sensible advice but miss some thing very obvious.
:-)
Please share?
Cheers
Davie
“A truly great library contains something in it to offend everyone.”
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Jasongray5
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| posted on 31/1/11 at 01:15 PM |
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My plan was to earth the engine to the chassis, then have a single earth to the battery, then another fuse with a lower ampage on the positive side of
the battery, where it goes to everything else and not the starter, if that makes sence?
Sorry for any confusion
How hard can it be?
http://www.flickr.com/photos/33261515@N03/sets/72157611049241239/
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daviep
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| posted on 31/1/11 at 02:00 PM |
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quote:
Originally posted by Jasongray5
My plan was to earth the engine to the chassis, then have a single earth to the battery, then another fuse with a lower ampage on the positive side of
the battery, where it goes to everything else and not the starter, if that makes sence?
Sorry for any confusion
Perfectly acceptable in my opinion.
Regards
Davie
“A truly great library contains something in it to offend everyone.”
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