3 phase supply to single phases only 2 phases used

[catdog1121]

I went to some holiday cottages today, the main supply in was 3 phase however only 2 of the phases were used. The L1, L2 and Neutral cables were wired into a three phase meter, then coming out was L1 and Neutral, L2 and Neutral to create two single phase supplies which were divided off using 60amp isolator fuses to each of the cottages with the load roughly balanced over the two phases.

My understanding of 3 phase has only come from large datacenters where although the equipment mainly just used single phase it was always important to try to balance the load over the 3 phases as you got charged based on the peak phase.

Given L3 is not being used does the three phase meter just charge over the 2 phases used or would it be better to have the third phase connected and the load balanced over the three phases.

 

[PEG]

I think,to get to the bottom of this,we need a cottaging expert...

 

[Diytim]

Did you check if it was three phases coming in? Some farms were fed from a split phase transformer. Look and see if the HV line is two wire or three wire. Check the voltage between L1 and L2 around 500v indicates split phase.

Hi I have a customer on an old farm supply. Many other electricians have assumed it was just 2 of 3 phases supply but it’s not. I learned about farm supplies from my tutor at technology college 50+ years ago who worked for the local supply company.
They used the 2 phases for farm machinery which required a high load so used a 480 v supply but then had a normal 240v supply for the house. The 2 phase supply is just 2 x 240v supples working in opposition to each other. i.e. 180 deg out of phase.
my customer wanted to run some new 3 ph plant but the cost of installing a new 3 phase supply was extortionate so I installed an inverter for him and put in a standard 3 phase dist board. Work a treat and now have a customer for life ! !

 

[Lucien Nunes]

2 phase supply is just 2 x 240v supples working in opposition to each other. i.e. 180 deg out of phase.

We should really call it 'split-phase' not 2-phase, because it's just a single-phase 480V supply with a grounded centre-tap as the neutral. There was such a thing as 2-phase, with the phases 90° apart, used from the very early days of AC. Like 3-phase it has an inherent sense of rotation that makes induction motors self-starting, and an absence of power zeroes hence lower torque ripple, but 3-phase offers yet more versatility and economy and became the polyphase system of choice. Split-phase does not offer these advantages, only the ability to deliver 240V and 480V from a 3-wire supply with less copper than would be needed with 2-wire circuits.

 

[Diytim]

We should really call it 'split-phase' not 2-phase, because it's just a single-phase 480V supply with a grounded centre-tap as the neutral. There was such a thing as 2-phase, with the phases 90° apart, used from the very early days of AC. Like 3-phase it has an inherent sense of rotation that makes induction motors self-starting, and an absence of power zeroes hence lower torque ripple, but 3-phase offers yet more versatility and economy and became the polyphase system of choice. Split-phase does not offer these advantages, only the ability to deliver 240V and 480V from a 3-wire supply with less copper than would be needed with 2-wire circuits.

One problem I got an emergency call out on a strong windy night where lights were going up and down and dist board smoking. On investigation I found we had lost the neutral supply and 480v was being applied to the household single phase circuits. Had to pull the main cut out fuses and inform the electrical supply company. Next day they found a branch from a tree Had fallen across the overhead neutral cable from the pole mounted transformer and severed it The supply company paid for a electrical contracting company to replace the house dist board and buy all the appliances which had blown up! They didnt‘t like 480v up them.

 

[Lucien Nunes]

I don't know which is more damaging, 400V from a broken neutral on a 3-phase system with highly unbalanced loads or 480V under the same conditions on split-phase. 480V might be more spectacular in some cases, but the higher the voltage, the more likely it is to blow equipment fuses etc. A longer, lower overvoltage might cook transformers that would otherwise just pop their fuses.

I have seen (and been obliged to repair) lots of studio and electronic kit that has seen 400V due to 'user error' normally on temporary supplies via single-pole connectors such as Cam-Lok or BAC. Either neutral loss or neutral crossed with a phase, which at least assures that things will actually get smoked rather than just go wibble.

I am not blameless in this regard, having once plugged up a set of 400A BACs in semi-darkness and got the blue and black in the wrong holes. I put the main breaker in with the intention of checking the voltages as one always does, but overlooked a 10kW tungsten studio light connected to red phase that was already switched on. It lasted one second, then took 5 minutes to sweep up.