my building has had a 'phase' of the electricity supply fail and some fuses/circuits still operate and others do not - is that one of the tri phases - they call it L1
Yes, you have described a classic "phase failure" (also known as a lost phase or single-phasing).
In a three-phase electrical system, the supply is delivered via three separate "hot" lines, typically labeled L1, L2, and L3. Since large buildings and industrial sites distribute their various single-phase circuits (standard wall outlets, lights, etc.) across these three lines to balance the load, losing just one-in your case, L1-causes approximately one-third of the building's standard equipment to lose power.
Why some things still work
The power in your building is distributed like this:
Single-Phase Loads (120V or 230V): Standard appliances and lights are connected between one of the phases (L1, L2, or L3) and a common Neutral wire. If L1 fails, only the items plugged into the L1 circuits go dark. Items on L2 and L3 remain fully powered.
Three-Phase Loads (Motors/HVAC): Large equipment like elevators, industrial AC units, and pumps require all three phases to create a rotating magnetic field. If L1 is missing, these machines will either fail to start or-if they were already running-will begin to vibrate, overheat, and potentially burn out because they are trying to do the same work with only two-thirds of the required input.
Common Symptoms of a Lost L1 Phase
Symptom
Cause
Partial Blackout
Roughly 1/3 of the lights and outlets in the building are dead.
Humming Motors
Large motors (pumps, fans) may hum loudly but won't spin.
Dim/Flickering Lights
Some devices may experience "backfeed" through three-phase equipment, receiving a low, unstable voltage.
Tripped Breakers
The main breaker or specific "phase failure relays" may trip to protect the system.
Is it dangerous?
Yes, it can be. While a lost phase isn't usually an immediate fire risk for your lights, it is highly damaging to three-phase machinery. If a large motor continues to run on two phases (single-phasing), the current in the remaining two lines (L2 and L3) can spike to 1.7-2.4 times the normal level, leading to rapid overheating and motor failure.
What should you do?
Turn off large equipment: If you have control over any three-phase machinery (central AC, large pumps, or shop tools), switch them off immediately to prevent motor damage.
Check the Main: Sometimes a single pole of a three-pole main breaker has tripped.
Contact the Utility/Electrician: If the issue is at the street level (a blown fuse on a pole or a downed line), only the utility provider can fix L1. If it's internal, an electrician will need to check your main distribution board.
Would you like me to help you draft a description of the symptoms to send to your building manager or utility provider?
This video provides a clear visual explanation of how a three-phase motor behaves when one of the phases is lost, which explains why some of your building's equipment might be struggling.
Understanding Phase Loss in Motors
