3 phase advice needed urgently

Thread Starter

Bazzo

Joined Jun 27, 2018
8
A little advice needed.. Electrician coming back for 5th time tomorrow
I live in Spain.. And think I'm being told a load of rubbish by a supposed electrician.. Lots of fakes over here!!
So.. I have 3 phase to the house.. 3 live, 1 neutral 1 earth. Each phase is used independently
1 phase runs the house, one phase runs workshop 1 and 1 phase runs workshop 2 a circuit that is hardly used. Now it keeps blowing the 80A fuse on the neutral in main incoming board. Am I right in saying that's because the 3 phases are imbalanced.. House pulls about 8kw, workshop 1 about 8kw workshop 2 about 1kw.. Electrician tells me that it's because I need more neutrals.. Which I know is bollox... Possibly a bigger neutral due to imbalance.. But I would have thought the correct answer is to balance the phases out?.. I'm not an electrician but I am a qualified electronics engineer in industrial field.. Retired now though...

Any advice would be much appreciated.
 

Hymie

Joined Mar 30, 2018
877
A little advice needed.. Electrician coming back for 5th time tomorrow
I live in Spain.. And think I'm being told a load of rubbish by a supposed electrician.. Lots of fakes over here!!
So.. I have 3 phase to the house.. 3 live, 1 neutral 1 earth. Each phase is used independently
1 phase runs the house, one phase runs workshop 1 and 1 phase runs workshop 2 a circuit that is hardly used. Now it keeps blowing the 80A fuse on the neutral in main incoming board. Am I right in saying that's because the 3 phases are imbalanced.. House pulls about 8kw, workshop 1 about 8kw workshop 2 about 1kw.. Electrician tells me that it's because I need more neutrals.. Which I know is bollox... Possibly a bigger neutral due to imbalance.. But I would have thought the correct answer is to balance the phases out?.. I'm not an electrician but I am a qualified electronics engineer in industrial field.. Retired now though...

Any advice would be much appreciated.
From the limited information supplied, I would imagine that you have fuses in each of the 3 phases (<80A), with a fused neutral.

I would think the most likely scenario is that you have an overcurrent fault in one of the phases (live to neutral), but combined with the neutral current from the other phases, is sufficient to blow the neutral fuse before the phase fuse.

My advice is to check the phase fuse ratings (and the fuse characteristics) to understand why one of the phase fuses is not operating before the neutral fuse.

An imbalance of phase current should not cause the neutral fuse to blow.
 

MaxHeadRoom

Joined Jul 18, 2013
21,605
If this uses a ground neutral it should Never normally be fused.
Running a star N three phase transformer unbalanced should not be a problem.
If all are single phase supplies/loads, each supply will be single phase whether fed from just two phase or a phase and N.
Any supply using the N, this N should be sized appropriately for this particular supply.
M.
 

Thread Starter

Bazzo

Joined Jun 27, 2018
8
OK.. Thanks.. I'll try and isolate a problem...
But am I right in saying that only 1 neutral is needed.. And only needs to be same size a lives... I thought that if phases were balanced then on a very small current would flow through neutral?

If I take my Amp clamp meter and measure the current draw on each live, how close should they be in an ideal world?. Within 10A of each other?
 
Last edited by a moderator:

MaxHeadRoom

Joined Jul 18, 2013
21,605
Yes one neutral per service, and should be sized same as the phase conductor, if the transformer is balance the neutral current will be less.
Max.
 

Thread Starter

Bazzo

Joined Jun 27, 2018
8
Forgot to add... All. 3 phases and neutral have 80A fuses in main board.. Then all 3 phases go to 63A breakers.
 

Hymie

Joined Mar 30, 2018
877
Forgot to add... All. 3 phases and neutral have 80A fuses in main board.. Then all 3 phases go to 63A breakers.
Imagine that you have a 3 phase supply, each phase rated at 8kW. If only one phase were fully loaded at 8kW (with no load on the other phases), then the live to neutral current would be around 35A.

But because of the voltage phase differences, loading the other phases results in some of the current flowing between phases rather than to neutral. Once the phases are balanced, the neutral current will be zero, even when each phase is fully loaded at 8kW.
 

Thread Starter

Bazzo

Joined Jun 27, 2018
8
OK. That makes sense.. Worst case scenario maybe house only pulling 1kw or so.. Workshop 1 can sometimes pull 10kw and workshop 2 is sometimes pulling nothing... But even I'm that scenario it's still only about 40A...
Can someone confirm that the neutral is always same size as the phases.. And that there is only ever 1... I'm pretty sure about this but the "sparky" tells me I need another neutral..... Looks as though there is a fault somewhere though...
 

MaxHeadRoom

Joined Jul 18, 2013
21,605
If any 1ph circuit supply is fed from one phase and the star neutral, both conductors should be sized equally, regardless of what the phase current is.
Max.
 

Hymie

Joined Mar 30, 2018
877
If your electrician fails to offer a sensible fix – I would consider replacing each of the phase fuses with a lower amp rating (say 30, 35 or 45A). Then whichever phase is supplying the fault current (to the neutral line), should operate the phase fuse before the neutral fuse.

Given that fuses can withstand 150% their rated current for a considerable time, a 30A fuse should not nuisance trip.
 

Thread Starter

Bazzo

Joined Jun 27, 2018
8
OK.. Thanks for all the replies.. So I'm not mad.. Cables should all be same size and only 1 neutral... I do have a clamp meter... Will measure them all on full load and maybe change fuses..
 

Hymie

Joined Mar 30, 2018
877
OK.. Thanks for all the replies.. So I'm not mad.. Cables should all be same size and only 1 neutral... I do have a clamp meter... Will measure them all on full load and maybe change fuses..
If your clamp meter has a peak hold function, you could leave it monitoring on each phase in turn for an extended period to catch the phase current causing the issue.

However your clamp meter may power-down automatically after a set period, and the peak measurement function may not have a fast enough response to catch the short period fault current. Often the main purpose of the peak hold function is to allow a reading to be taken – with the meter in a location where the display is obscured.
 

Hymie

Joined Mar 30, 2018
877
Spain doesn't have an earth grounded Neutral?
If it does it should never be fused.
Max.
Yes, I too do not understand why the neutral is fused.

Once this fuse blows, the neutral voltage could rise due to the current imbalance on the phases (potentially causing all sorts of issues for the connected powered equipment).

I’m not aware of anything unusual about the wiring system in Spain – and therefore would expect no fuse in the neutral, and the neutral referenced (connected) to ground/earth at some point.

In know that in some parts of France the supply system is isolated from earth – but nevertheless an earth connection is provided.
 

Thread Starter

Bazzo

Joined Jun 27, 2018
8
I did wonder the same.. I am pretty sure he added that fuse about 6 months ago. So the neutral should be bonded to the earth and not fused?

In case you are wondering why the "electrician" is even in the main incoming board... Here it can take weeks for the supply company to visit even on an complete outage.. It's common practice for an electrician to cover everything right back to the main incoming fuses..
 

Thread Starter

Bazzo

Joined Jun 27, 2018
8
Yes. Think that's next... But I have paid him a lot of money and it still isn't right... He got 5w emergency lighting circuit on a 33A breaker.. A 5kw load on a 20a breaker.. Mains in on 2 of the distribution boards are 100A switches instead of breakers.. One of the phases only has an RCD and no breaker whereas another phase has RCD and breaker... One intermediate breaker board has the live in and live out connected to one terminal on the breaker as it has burnt out at some time... Just so many things wrong I've found now I've taken time to look at everything
 

Hymie

Joined Mar 30, 2018
877
I did wonder the same.. I am pretty sure he added that fuse about 6 months ago. So the neutral should be bonded to the earth and not fused?

In case you are wondering why the "electrician" is even in the main incoming board... Here it can take weeks for the supply company to visit even on an complete outage.. It's common practice for an electrician to cover everything right back to the main incoming fuses..
The neutral of the electrical supply system should not be fused.

Although the neutral will be referenced (connected) to earth as some point, normally this connection is at the sub-station or possibly at the boundary of the supply entering the property.

If you were to simply link neutral and earth at your consumer unit (main incoming board), this will likely cause any ground fault interrupters (earth leakage) to trip – with the supply current returning to earth rather than through the neutral line.

So unless you know what you are doing – great care should be taken in grounding/earthing the neutral; certainly not a job for your current electrician.
 

MaxHeadRoom

Joined Jul 18, 2013
21,605
I know nothing of the requirements in Spain and I am not sure if anything has changed in the UK since I practiced there, but when I did, the neutral was grounded at the distribution transformer star point and never at the customer service entry like it is here in N.A..
The service supplier would refuse to connect if the neutral was connected to the earth conductor at entry or anywhere in the installation.
Max.
 
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