Cable current carrying capacity

[Abe]

Ill tell you why I chose Peters method........even before I go 2 moro to sort it

Peters looks to me the safest option........

I have all the circuit breakers and isolators.........

Cable will be 2.5 mm size

Ill take all the readings and post 2 moro

[Abe]

And its not 2.5 mm for all 6 heaters.....

Its one 2.5mm per phase

[chillin out]

Abe,

Be adventurous and go for the other option. You can then experiment and tell us how it goes.

(you do have insurance don`t you?)

[Peter_1]

No doupt that the best way to do this job is to wire a neutral , but that doesn't mean that other method don't work , and other methods can have protection devices againt incidents caused by a heater fail

Sure, other methods can work fine.

We even installed safety motor current trippers instead of a fuse. So you can dial in very precise the current.
We then use the aux contact to trigger an alarm.

Same for protecting a transformer.

Regarding the current in the neutral due to an earth leak: in Belgium, the earth leak protetcor must cut-out when there is a leak of more than 300mA (in many cases even 30 mA)!, so this is not a big current.

When they're wired in serie - and suppose you wired the 2 drain pan heaters together - and one heater of the 2 fails, then all the water in the drain pan will become in ice, even so worse that it will push the drain pan away (seen it many times)
This isn't unsafe but you've inserted a possible fault that could give serious troubles.
We try to avoid possible errors.

In Belgium, it's not obligated to run the neutral over the contactor nor the main heater fuse. It may connected straigth to a 'neutral bar'
We never do this for the following reason: if you have an earth leak and all the neutrals are wired together, then it's almost impossible to find the eartleak. You have to losen all the screws on the bar beacause if you megger the circuit, the fault is everywhere and nowhere.

In an unbalanced system 3 phase system, there is always more current flowing through the live wires then the neutral but this doens't mean that there is an earth fault.
The example with the simulation shows this perfect.

We never calculate the AMPS for the neutral, just use a 5 wire cable.

Abe, it's indeed 2.5 mm² for every wire.

Another thing, we allways install a mechanical safety thermostat on the roof of a freezer and don't tell the client it's there. We dial in a fix setting and remove the knob of it.
We already had serious damage due to a failing electronic end-defrost systems.
We once had 7 ton of chicken meat in a poultry quick freezer which went to 95°C (!!) during the weekend.
Happily we were insured for this.
It was more a cooker then a freezer.
And once a freezer with +/-1.500 pastries in it in a bakery freezer.

[frank]

Sure, other methods can work fine.

So what you are saying then Peter is that, with the correct safety measures in place, a 3 phase multi defrost heater installation can work just fine without a neutral connection?

[Peter_1]

If you search once in the history of this forum, you will find exactly the same discussion.

The theory behind all this is the Law of Kirchoff and Marc O'Brien mentioned also the Thévenin Theorem which I forgot completely.

It all depends how many heaters are connected to the 3 phase system.
If more then 3 and some are connected in parallel without a neutral wire, then this is for the heaters - not the user - an unsafe situation.

Sure, connecting two in serie on 1 line is not unsafe as long as you don't connect heaters in parallel.
But connecting them in serie makes the installation more failure sensitive (if this is the correct English expression)

Other safety devices which can be used: overvoltage arresters, varistors, ... These devices works also fine but these are expensive solutions if you can use instead the (cheap) neutral wire.

We sometimes use varistors - which are very cheap - for the SV's in market trucks to protect them against overvoltage

Has nobody encountered in real life problems when the neutral is sudden lost along the electricity lines (outside your house)?

It was in the news some weeks ago but a worker here very nearby had in a transformer house disconnected the neutral accidendaly and +/- 160 houses had serious failures on electric components.

Still haven't got any other answers for the moment from manufacturers.

But why not try somebody this in theorey with some light bulbs and convince yourself? I have a strong feeling that this is something that most of us aren't aware of.
Take 8 bulbs of 220 VAC in 8 cheap sockets and connect them to a 3 phase 380-400 V power supply and see what happens.
Take voltage and AMP readings on the lines.
An example shows sometimes more than 1000 words.

[Peter_1]

Dear Sir,
The elements connected in our coolers are always connected in star with a neutral line.

The reason is what you already say in your mail.


Some people connect the elements in our cooler without the neutral.
The number of elements is than always divided by 3.
If, in this case, one of the elements fails, the problem of unbalance arise.

Our "professsional" advice is, Use a neutral.

Best regards

Digni van der Zande
Office Sales Manager
Goedhart Cooling Equipment

[frank]

If, in this case, one of the elements fails, the problem of unbalance arise.

As I've said before

with the correct safety measures in place

[Peter_1]

Anyone tried the set-up with the light bulbs and unscrewing some to see what happens with the voltage?

[chillin out]

Going by Franks wireing diagram, would you really get 240 v between phase and star-point?

If you think about the wave form of 3 phase, and then think about what you are doing with it.

When you measure between 2 phases you should get 415v , but then you introduce a third phase, this is out of sync with the other 2 by 120deg giving you 180v between any phase and star-point.

I think!!!!!

just throwing in some ideas.

[botrous]

The star point is a neutral point , the voltage between the start point and any of the phases should be equal to voltage between 2 phases devided by the square root of 3...

Regards

[Peter_1]

http://en.wikipedia.org/wiki/Three_phase

http://www.du.edu/~jcalvert/tech/threeph.htm

This is a good one:
http://www.electrician.com/electa1/electa4htm.htm

http://www.electrician.com/electa1/electa5htm.htm

http://www.electrician.com/electa1/electa3htm.htm

[chillin out]

All of those links seem only to be concerned with currant and not voltage, which is what im talking about.

botrous.. I know the star-point is called 'neutral' but its not an actual no volts dead spot is it?

I will continue my googling.
Any points to prove/dissprove me are welcomed.

[Abe]

All six heaters connected. 2 heaters per phase off a three phase supply
2 X 1500 watt heaters connected per phase
All of one 18 Amp Contactor


These are the readings.

12 amps draw per phase, so Im assuming each heater is drawing 6 amps

Each phase protected by a 16 amp type B circuit breaker and a 20 amp cartridge fuse at main Distribution box

Neutrals are ALL linked into one bus bar.

Ill go to site again next Wednesday to see if Evaporator is clear

I need to ask one other question

The drain line heater is permanently on and gets quite hot.
It is not self regulating.

Is that safe??

[Peter_1]

All of those links seem only to be concerned with currant and not voltage, which is what im talking about.
botrous.. I know the star-point is called 'neutral' but its not an actual no volts dead spot is it?
I will continue my googling.
Any points to prove/dissprove me are welcomed.

Chillin, via the Law of Ohm, you can convert this to voltage
The star point in NOT always A neutral point and certainly not THE neutral point. It's only neutral in a balanced field. In a balanced set-up it's indeed a 0-volt point and you don't have to connect it, ...like in an electrical motor where all 3 windings, so the load are equal.
Prove/dissaprove...go to the nearest electrical shop and buy for 5£ of material and a practical test shows more then 1000 words. But as I said previous, try to understand why this happens.
Google on Kirchoff and Thévenin.

[botrous]

In a balanced set-up it's indeed a 0-volt point and you don't have to connect it, ...like in an electrical motor where all 3 windings, so the load are equal.

Peter , you made my point .
Chillin out , that's what I meant to say

[frank]

When you measure between 2 phases you should get 415v , but then you introduce a third phase, this is out of sync with the other 2 by 120deg giving you 180v between any phase and star-point.

Hi Chillin
What you are seeing is the third phase acting as a neutral due to the phase shift. If you only have 2 phases requiring a neutral then a wire is needed. But with a balanced 3 phase load you do not need a neutral wire as one phase always acts as a neutral current path.

Have you ever noticed that you can measure voltage upto the load but only current from the load?

[chillin out]

Thought about it and found that all I have to do is go to a comp which is wired in star (arctic circle as the star point is accessable) and measure it there, will do that tomorrow.(unless I get called out again tonight )

Have you ever noticed that you can measure voltage upto the load but only current from the load?

This true on a 2 phase motor?

[Peter_1]

Between each fase and the copper star-bridge, you will measure 220 V.
Measure once between the bridge and earth (the copper lines connected to th compresser -->O V) and between a fase and the copper of the lines connected to the compresser (again 220 V)
Do you understand what's happening?

[chillin out]

I think all is clear now, went to a site today and took measurements. 240v between phase and bridge.

Thanks all,

[Peter_1]

What was the reading between line and ground and between bridge and ground?

If you haven't measured it, what should the reading be?

[chillin out]

Didn`t measure it,

between line and ground

240v

between bridge and ground

0v

[Peter_1]

You got it and I'm sure this is something you will now never forget anymore.
If you have some time, do once the test with the bulbs, you will learn even a lot more.