Floating Neutral with Single Phase Inverters in machinery

Question

Hi, As a new member I thought I would throw out my first question from a recent experience. 
 
 I had a customer who bought a machine which had Single phase supplied servo motors & a Single phase Supplied Inverter. Each Servo drive was supplied by a phase 
 Servo 1 - L1 - N 
 Servo 2 - L2 - N 
 Servo 3 - L3 - N 
 Inverter L Something to N. 
 These where protected by Type C MCBs in the control cabinet. The machine had a 3Pole Isolator only isolating the Phases. 
 
 ISSUE. 
 The Electrician had wired a new 32a 5 Pin Plug to supply the machine. Upon commissioning the machine our engineer had done a voltage check & phase rotation check at the supply coming into the machine which. had tested 230v L1-N, L2-N L3-N. 400 v L1-L2. L1-L3. L2-L3. 230v L1-E, L2-E L3-E 
 As far as the engineer was concerned the correct voltages tested out ok and the next step was to turn on the machine. 
 
 Big Puff of smoke which took out the cooling fan, 24v PSU & single phase inverter. 
 
 When we looked into the issue, it turned out the electrician had not wires the neutral in the distribution board assuming they only needed 3 Phase. Checking the voltage between Line & Neutral gave us almost 400v when the machine was on. It appears the voltage supplying the Servo Drives had no where for the current to go with the disconnected neutral and created a potential through the neutral providing all the single phase devices with 400V. However when the machine was switched off, the neutral was not switched and a return path must of been created through the drives or PSU to Ground. 
 
 Question 
 What are the regulations on protecting single phase drives with using more than 1 phase in the same cabinet & what devices should be used to protect against this from happening? 
 
 Thanks 
 
 Symon

broadgage · Accepted Answer

In my view, installing a FIVE PIN Ceeform outlet without a neutral is simply wrong. May not be specifically prohibited by regulations, but what about common sense, or accepted practice ? It is nearly as bad as fitting a 13 amp socket without an earth because only double insulated appliances are to be used. 
 If no neutral is required then the four pin type should be installed.

Zoomup · Answer

And, the installer of the machine should have read the manufacturer's installation requirements for the machine, and also noted the machine terminal markings and supply needs. Was there a neutral terminal in the main terminal box? We may all have been fooled by the lack of Neutral at the supply socket if it had an apparent Neutral provision, and the supply Voltages seemed good. Testing at the machine with a neon Voltage tester or a sensitive digital multi meter may have not revealed the true picture. A tungsten test lamp or a small load may have revealed the lack of a Neutral as Mike has detailed above. An artificial internal Neutral may have been created by internal filters or other components. 
 
 Z.

Zoomup · Answer

With no N. connected inside the machine, Voltages will vary and may reach up to 400 Volts on the individual L1, L2 and L3 terminals. The connected N. provides an essential reference point. Correct wiring should protect the machine from damage. No regs. required. 
 Edit. Add. 
 Three Phase Mystery Solved, No Neutral Required in a Balanced Load? - Bing video 
 Z.

AJJewsbury · Answer

Lost N causing a overvoltage on single phase connections is a well known problem - probably one of the reasons behind the preference for 3P rather than 4P switching for 3-phase, where leaving the N solid is permitted. 
 Other than the requirement for each circuit to have its own individual N, I don't know of any regulation that's intended to prevent this sort of situation - it's certainly permitted to omit N where it's thought not to be needed. Likewise there's no stipulation that 2300V single phase loads shouldn't be connected across phases - I guess a degree of common sense is presumed. 
 I am a little surprised that L-N voltages appeared correct if no N was supplied - unless there were some other loads connected on the feed side of the isolator and some of the N conductor was present. 
 The much worse situation is where the N also serves as a PE conductor (CNE or PEN) - in which situation you can find exposed-conductive-parts sitting at 230V indefinitely and even RCDs won't trip to save the situation. 
 - Andy.

mapj1 · Answer

More accurately, the correct voltages were generated by a star of near identical impedances - with equal currents from all 3 phases to a floating centre point there is no need for a neutral conductor to get the correct 'zero-centre' . And when all 3 are on standby, the current through the mains filters and wiring capacitance is presumably near equal enough to fool you. But as soon as one of the 3 loads is not quite the same as the other two, it all goes wrong, as without a neutral, the most heavily loaded phase sees a falling voltage, the star point moves closer to that corner of the triangle if you like, and in true comedy see-saw fashion, the other two phases are then horribly over-volted. Of course as the voltage changes, so does the current drawn, and some loads will disconnect more or less impressively as the voltages go too far out of bounds, so the effect can be a few seconds of destructive psychedelics of flashing lights and motors buzing and stalling an then restarting and things blowing. To test for a high impedance neutral, having passed the balanced with no load test then put a load you do not care about too much onto one phase only - a cheap electric heater element is a good candidate. and see if the 3 phases remain more or less equal voltage. IF they go off piste, check those connections before switching on the expensive stuff. 
 Mike.

Floating Neutral with Single Phase Inverters in machinery

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