time limit fuses

[jimmy2times]

this is a homework question but as I've found no info on the net I hoping some people familiar with this older technology can help out.

In what application, or when were, time limit fuses preferred over other solutions for protection. Very little on the net in discussing application of this legacy technology which may still be in place in some installations.

Any help appreciated even if a link to further reading material.
thanks

 

[ScottyUK]

Is this the scheme which had a fuselink across the coil of a current-operated relay and the fuselink blew after a current-dependent delay? I can't quite work out from the description what the scheme is, or maybe it just wasn't adopted in the UK. The UK was Great Britain back in those days!

 

[jimmy2times]

Spot on scotty. I understand a bit about their operation, just not sure where they were favoured. Was it just the ease in coordinating devices by simply selecting appropriate fuse that made then favourable over idmt relay. Or was it particular application that they were optimum choice for. I've found literature relating to there use in ring main units as an option. Scratching around for info otherwise

 

[ScottyUK]

My opinion is that they allow use of a relatively cheap attracted armature relay and cheap CTs to give an inverse time characteristic, probably an EI curve in modern relay terms. The accuracy isn't great, especially at low multiples of current, and they have a limited range of adjustment with no option for refinements suchs as a DMT setting or instantaneous element or a different response curve. They are low maintenance compared to an induction disc relay and require no calibration, and they are reasonably reliable. That feature set places them in the high volume, low cost, low criticality area of the market, e.g. RMUs and the like.

 

[Marmite]

Time limit fuses are far from legacy protection. They are still in widespread historical and current use on the distribution system in the UK. They are used to protect the HV winding of a ground mounted distribution transformer upto around 1500kVA. These days they are predominantly installed in ring main units, or extensible switchboards at distrubution level to protect transformers. The main benefit is that they are part of an AC protection system, so there is no need for a tripping battery and charger. The distribution network operators have each got 1000's of indoor ground mounted substations and to have a tripping battery in each would be a massive ongoing maintenance liability. See

http://download.schneider-electric....cId=&p_Reference=SE6251&p_EnDocType=Technical

leaflet&p_File_Id=1612230&p_File_Name=SE6251.pdf

There is a schematic diagram in the Schneider document showing the principle of operation. It is about as simple as it gets.
Some DNO's install a TLF in the earth fault element, which you may question the need for, when you are protecting a delta winding, but it does give positive evidence of a fault. If a TLF blows it is useful to break it open to see if the element has melted to rule out another mode of failure/cause of tripping.
The TLF is usually graded with low voltage fuses, which may be 400-600A. If the substation feeds a customer with their own HV network, or their own transformer, then the protection would be via self powered relays rather than TLF's.

Regards
Marmite

 

[waross]

In smaller sizes, up to about 60 Amps at 208V, 240V, 480V or 600V, dual element fuses were used for problem motor starting applications. If the normal fuses were blowing on motor starting currents, the issue could often be resolved by installing dual element fuses. These combined a high rupturing capacity instantaneous link in series with a thermal link. The thermal link had a time/current characteristic that would withstand motor starting surges but melt on sustained overloads. The instantaneous link would clear fault currents. Properly sized these would provide backup protection for the motor overload relay, and were marketed for motor protection.
This gave better and cheaper protection than changing out the disconnect switch for a higher rated switch. In some instances changing to a higher rated switch may require the conductor and conduit sizes to be increased.
In initial installations a smaller disconnect switch was often adequate.
In the event that a motor was to be replaced with a larger motor, the use of dual element fuses would often avoid the need to change out the disconnect switch to a larger size.

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

 

[jimmy2times]

Thanks for help and useful attachments