# Aren't fuses a bad idea in multiphase circuits?



## José_Fuentes (Aug 8, 2014)

I have seen fuses used to protect 3-phase primary distributions systems. However, it seems to me that for most faults, only one fuse will open (phase-to-neutral fault) and so it will leave 3-phase transformers on a bad condition. Why do then power companies use fuses in multiphase primary circuits?. Is it that they don't care about single-phasing their transformers risking damaging them and the connected customers?. Regards and thanks.


----------



## Black Dog (Oct 16, 2011)

José_Fuentes said:


> I have seen fuses used to protect 3-phase primary distributions systems. However, it seems to me that for most faults, only one fuse will open (phase-to-neutral fault) and so it will leave 3-phase transformers on a bad condition. Why do then power companies use fuses in multiphase primary circuits?. Is it that they don't care about single-phasing their transformers risking damaging them and the connected customers?. Regards and thanks.


Fuses do a better job and the POCO's don't care about your equipment.


----------



## ponyboy (Nov 18, 2012)

It's much more difficult to blow a fuse on the utility side. You can't even think of it in the same way you think of our standard low voltage fuses. Continuity of service is a primary concern for utilities, not personal and equipment protection


Sent from my iPad using Tapatalk


----------



## José_Fuentes (Aug 8, 2014)

Black Dog said:


> Fuses do a better job and the POCO's don't care about your equipment.


 But doesn't the power company cares about its own equipment? As far as I know, leaving a phase disconnected to a 3-phase transformer may cause damage because of the inbalance of the magnetic flux (I have read of tank heating in this condition because the tank becomes part of the magnetic circuit, since the magnetic flux in the legs do not add to 0).

Anway, is supplying only 2 phases to a customer allowed by some juristiction (Say, the US)?.



> It's much more difficult to blow a fuse on the utility side. You can't even think of it in the same way you think of our standard low voltage fuses. Continuity of service is a primary concern for utilities, not personal and equipment protection


 Is there some difference apart from the current rating, thermal capacity (also known as _thermal mass_) and that some are resettable/reusable?.


----------



## Tarmiito (Feb 18, 2014)

As far as i have seen, fuses are mostly used for to-low-voltage transformer protection. Although blown fuse will leave the three-phase transformer in unsymmetric stage cousing under/overvoltage, it's still the cheapest/safest way to protect the transformer.
I've seen fuses used for line protection as well but it's a bit unresponsible for the consumers behind the fuses. No blackout for the lucky people before them though. 
I guess they probably use them a lot for single-phase mid-voltage lines. Seen them in Philippines.
We have in Estonia also a transformer switch, that uses fuses but switches off when a fuse is blown. Nice equipment, only if it wouldn't have the isolatsion problem in humid.
Something like this:


----------



## Meadow (Jan 14, 2011)

COST: Its cheaper to protect the line and to disconnect power via fused cutouts. 

{The other 2 options are a. microprocessor controlled switch or recloser (best at line protection) but costs at least 20x more; b. Use feeder breaker or inline recloser for all protection. Draw backs are increased conductor size on laterals and having outages take out many more customers} 



Downsides:

1. High impedance faults go unnoticed (such as a wire across black top)

2. 3 and 4 core wye grounded primary 3 phase units risk damage from inductive flux tank heating.

3. back feed into the downed conductor from the other phases.

4. Damage to 3 phase equipment.


----------



## Meadow (Jan 14, 2011)

Black Dog said:


> Fuses do a better job and the POCO's don't care about your equipment.


Rarely in my experience. Fuses offer some the worst protection next to not having anything.


----------



## wildleg (Apr 12, 2009)

fuses are fine for premises protection.

equipment that will be ruined from single phasing should have phase loss monitors and associated equipment, if necessary, to shut them down.

power company is only driven by money, and it appears that it mus be cheaper to do it the way they do.


----------



## Meadow (Jan 14, 2011)

José_Fuentes said:


> But doesn't the power company cares about its own equipment? As far as I know, leaving a phase disconnected to a 3-phase transformer may cause damage because of the inbalance of the magnetic flux (I have read of tank heating in this condition because the tank becomes part of the magnetic circuit, since the magnetic flux in the legs do not add to 0).
> 
> Anway, is supplying only 2 phases to a customer allowed by some juristiction (Say, the US)?.


 It cares to a degree. 

If the transformer is delta primary the risk does not exist with inductive tank heating. Similar with a 5 legged core designs and single phase units.

This in fact is the reason why US POCOs avoid 3 and 4 legged padmounts in addition to trying to use single phase units banked into 3. A 3 phase bank consisting of single phase units will not care for an open phase.

A delta primary is susceptible to ferresonance, however its less likely to occur at lower voltages like 11kv, and sufficient load on the secondary will prevent it from happening during a single phase condition. 




> Is there some difference apart from the current rating, thermal capacity (also known as _thermal mass_) and that some are resettable/reusable?.


[/QUOTE]

Fuses come with a time current curve and different speeds like K or T being very common in the US. 


If the fuse is coordinating with a recloser programed for fuse saving the heat dissipation loss after initial trip must be considered during the open intervals of the recloser and the fuse blowing curve of the reclsoer must let enough energy through to blow the fuse first after the fuse saving curve(s) have been exhausted. 

This has nice resources, trip curves and literature on protective application:

http://www.sandc.com/products/fusing-outdoor-distribution/default.asp


Trip curves 


http://www1.cooperbussmann.com/pdf/00000000-0000-0000-0000-000000000000.pdf


Others

http://www.cooperindustries.com/con...tems/resources/library/240_Fusing/R240302.pdf 



http://www.cooperindustries.com/con...tems/resources/library/240_Fusing/R240303.pdf


----------



## glen1971 (Oct 10, 2012)

In the area I'm in, there is a fair amount of fuses getting blow out by the winds.. No damage to the fuse, or holders, just gets shaken out and drops.. Most sites have an AC fail relay and will shut down all the motor loads once this happens. There are a couple of sites that have VSD's and the AC monitoring isn't effective while the drive is running - attributed to how the transformer is built (assumption)... So far no noticable damage has occurred...


----------



## guest (Feb 21, 2009)

glen1971 said:


> In the area I'm in, there is a fair amount of fuses _*getting blow out by the winds.. No damage to the fuse, or holders, just gets shaken out and drops.. *_ Most sites have an AC fail relay and will shut down all the motor loads once this happens. There are a couple of sites that have VSD's and the AC monitoring isn't effective while the drive is running - attributed to how the transformer is built (assumption)... So far no noticable damage has occurred...



I find that extremely hard to believe....I have some utility cutouts (courtesy of SCE during replacements/upgrades) and those fuses latch in HARD with a lot of force. There is no conceivable amount of "shaking" from the wind that can unlatch one without causing serious damage to the cutout, pole or other equipment. A faulty cutout that would let the fuse drop out due to vibration would end up with serious damage on the top contact from the huge arc that would be drawn as it fell open. 

The kind of momentary fault that will happen during wind conditions (tree contact, lines swinging together) can/will actually blow the fuse..but modern expulsion fuses will NOT show readily visible damage (burn marks, hanging fuse link tails, etc.) Often the ONLY visible sign from the ground would be the plastic weather seal cap missing. 

So either the fuses are getting blown out (most likely) or the cutouts are getting damaged bad enough to need replacement if they are allowing the fuse to drop due to shaking from the wind. :thumbsup:


----------



## just the cowboy (Sep 4, 2013)

*My POCO says thats squirrels do it*



glen1971 said:


> In the area I'm in, there is a fair amount of fuses getting blow out by the winds.. No damage to the fuse, or holders, just gets shaken out and drops.. ...


Had a fuse hanging from its holder on the xformer on the pole feeding my house. I asked the lineman when he came out why was it out of its holder and he said it must have been a squirrel, but I couldn't find any fried squirrels .


----------



## glen1971 (Oct 10, 2012)

mxslick said:


> *I find that extremely hard to believe*....I have some utility cutouts (courtesy of SCE during replacements/upgrades) and those fuses latch in HARD with a lot of force. There is no conceivable amount of "shaking" from the wind that can unlatch one without causing serious damage to the cutout, pole or other equipment. A faulty cutout that would let the fuse drop out due to vibration would end up with serious damage on the top contact from the huge arc that would be drawn as it fell open.
> 
> The kind of momentary fault that will happen during wind conditions (tree contact, lines swinging together) can/will actually blow the fuse..but modern expulsion fuses will NOT show readily visible damage (burn marks, hanging fuse link tails, etc.) Often the ONLY visible sign from the ground would be the plastic weather seal cap missing.
> 
> So either the fuses are getting blown out (most likely) or the cutouts are getting damaged bad enough to need replacement if they are allowing the fuse to drop due to shaking from the wind. :thumbsup:


There are no faults on the sites, and the utility just spent most the summer running a brusher down the right of ways, so that takes out the tree factor.. I don't recall seeing a fried squirrel adjacent to any of the sites I've found powerless.. Wind gusts in this area can typically hit 160 km/h (100 mph) and sustained winds of 90 km/h (56 mph) is not uncommon... Heavy wet snows also wreak havoc with the lines and those have resulted in lots of trouble calls for the utility folks...

I know that they went through the area about 8 years ago and replaced alot of their connections and fuse holders and the reliability increased dramatically.. If they see anything that is in need of replacement/repair while they are out, they make the call if it will last til a planned shutdown, or if it needs to be done while they are out on that call.. 

One last night had the wire to the top of the fuse broken off at the top of the holder.. I think most of those are attributed to age and winds... I'm not a lineman, but I like talking to them so that I can make my trouble calls to them make a bit more sense and also so I can understand them when they are talking about their equipment and when outtages may be needed..


----------



## DesignerMan (Jun 13, 2008)

I can't speak regarding fuses on main lines.
However, I always preferred fuses on the main disconnect to control panels I designed because of the higher SCCR ratings.
A MCCB cannot get close to the ratings you get with fuses.
As long as you have a AC monitor on the circuit a blown fuse won't cause any damage problems.
And most 3-phase motors will single phase on a blown fuse and trip the overload anyway...


----------



## dtmartin408 (Nov 11, 2012)

Electrical power distribution systems are designed and operated very differently than your electrical system in your home,office,or plant. Fuses are just a part of the protection on a distribution system. You will typicaly have a breaker/recloser with protective relaying at your substation feeding the circuit. Sectionalizing reclosers with protective relaying and fuses in the back bone and feeder portions. Then all of you laterals wether single or three phase will be fused. The goal is to provide continuity of service to as many customers as possible. Fuses are also one of the safest protective devises as long as they are cordinated correctly. But here is one of the main differences though. The utility does not use protective devices to protect their lines from overcurrent conditions but rather fault conditions. The only thing they will typicaly protect from overcurrent is their substation transformers due to the fact that they are one of the single most expensive pieces of equipment


----------



## Meadow (Jan 14, 2011)

dtmartin408 said:


> Electrical power distribution systems are designed and operated very differently than your electrical system in your home,office,or plant. Fuses are just a part of the protection on a distribution system. You will typicaly have a breaker/recloser with protective relaying at your substation feeding the circuit. Sectionalizing reclosers with protective relaying and fuses in the back bone and feeder portions. Then all of you laterals wether single or three phase will be fused. The goal is to provide continuity of service to as many customers as possible. Fuses are also one of the safest protective devises as long as they are cordinated correctly. But here is one of the main differences though. The utility does not use protective devices to protect their lines from overcurrent conditions but rather fault conditions. The only thing they will typicaly protect from overcurrent is their substation transformers due to the fact that they are one of the single most expensive pieces of equipment


Lines usually don't have to be protected from over load because thermal limits are almost always higher than voltage limits, plus they know what the peak load will be and where. So in theory you are correct, when an over current does occur its near 100% of the time a downed wire or failed equipment.


----------



## 3DDesign (Oct 25, 2014)

Good reason to install a phase loss monitor on the main disconnect in the building.
It's on the bottom of THIS PAGE


----------

