TIME LAG FUSE

Time Lag Fuse (TLF) - Also known as Slow Blow Fuse or Delay Fuse

Example of TLF

1. A fuse which will carry an over-current of a specified magnitude for a minimum specified time without opening. The specified current and time requirements are defined in the UL/CSA fuse standards.
2. TLF allows a temporary and harmless rush of current to move into the circuit without breaking open. Rather it opens on the sustained overloads and short circuits. Slow-blow fuses are ideal for circuits with a transient surge of power. This lets current to continue flowing intothe circuit at close to the fuse rating. This prevents the fuse from breaking open and having to be replaced. Usually, they are rated between 125% and 150% of the circuits' full load amperes.
   
   
3. TLF are used for circuits with high surge or starting currents, such as motors, solenoids, and transformers.
4. We tend to neglect or didn't realise the existence or importance of TLF, bare in mind that not all circuit breaker (CB) equip with TLF.
5. Most of older CB (more than 20yrs of age) using TLF to trigger over current relay, nevertheless there are still CB still applied TLF till to date, Tamco Cutler Ring Main Unit Type GR for example.
6. Since in our plant we have 48 RMUs from 34 Sub-stations and 45% of it using Tamco Type GR, I would like to share my experience with this particular RMU

7. 

   RMU - Tamco Type GR 

   Whenever  RMU trip, apart from inspects the circuit downstream, don't forget to check the TLF condition, otherwise you might facing impossible to CLOSED it later.
8. To check TLF you need to remove it from the mounting bracket, fail to do so will mislead you as the continuity test will shown continuity even to the blown fuse because the TLF circuitry is a "loop circuitry" 
9. Don't worry, you can touch the TLF on energized RMU as long as you open/touch only the isolated CB circuits. 
10. It is crucial to replace TLF with the correct rating, failed will create tripping condition beyond its operating scale.