Fourth pole on isolator

[JohnW2]

A four pole will open & close all four poles, but without early make/late break.

That's what one (at least, I) would expect, but rocky seemed to be talking about one in which the neutral was not switched - which is why I was confused.

This can cause voltages approaching 400V to appear on equipment. Basically if the neutral is lost then you could potentially have a fireworks display.

You're saying the same as rocky. Maybe I'm being dim, but I can's yet get my head around that. 400V (or 415V) between what and what?

 

[JohnW2]

The solid N link on the Ops one is to the left side of the poles it's just a flat bit of metal same as the earth.

I'm again confused. All I can see 'on the left' appears to be the earth (with a bit of G/Y tape). What am I missing?

As for the 415 I mention, we do fryers with 3 x 240 v elements and 240v electronic controls,, when all 4 poles try to switch together, I feel if the 4th pole in the isolater shown was a bit slow and used as N, loss of neutral can cause two phases to oppose each other via the elements causing up to maybe 415 volt between phase and N

As I've just written to Risteard, I've yet to get my head around that suggestion.

Furthermore, if I understand correctly, you and he seem to be saying opposite things. You are saying that can be a problem if the neutral 'breaks late' ("a bit slow"),whereas he seems to be say that (the same) problem arises if the neutral does not break late'!

We may need a referee!

Though isolaters when I was taught were not designed to use as switches, to switch under load.

I think that is theoretically still true, but I think you'll find that, these days, all things sold as 'isolators' are specced to be able to break the full rated current.
Edit: having said that, it seems, from the Wikipedia article to which RandomGrinch linked, that devices designed to isolate but NOT able to break high load currents are perhaps now called "Disconnectors"?

Kind Regards, John

 

[JohnW2]

Mr Google got me here (which seems an interesting resource):

Switch Disconnector 3P | Single Line Symbols

Switch Disconnector 3P - Single Line Symbols

symbols.radicasoftware.com

- a 3P switch disconnector.

Thanks.

...and with the aid of Mr. Wiki:

Disconnector - Wikipedia

en.m.wikipedia.org

I would suggest that the symbol refers to the entire isloator and not just the neutral switching.
The switch is also for off-load isolation of the appliance, with normal operation and shutdown switching, provided elsewhere (which would also make early make/late break on the neutral unnecessary?)

Thanks again. It seems that what is now being called a "Disconnector" are things which rocky and I were brought up to regard as "Isolators" (i.e. not designed to switch under load).

Kind Regards, John

 

[SUNRAY]

Indeed and I'm convinced a disconnector used to be an 'on load' disconnecting device and was always twice the size of an isolator.

 

[JohnW2]

Indeed and I'm convinced a disconnector used to be an 'on load' disconnecting device and was always twice the size of an isolator.

So that's the opposite of the way in which the word is now apparently being used?

Kind Regards, John

 

[plugwash]

You're saying the same as rocky. Maybe I'm being dim, but I can's yet get my head around that. 400V (or 415V) between what and what?

If the nuetral disconnects before the phase conductors, then high voltages can develop between the now-floating neutral and one or more of the still-connected phase conductors.

Exactly what the voltages will depend on the characteristics of the load. If the load is nicely balanced the neutral might be removed and things continue as-if nothing had heppened. but if one or two of the phases are lightly loaded while others are heavilly loaded then the lightly loaded phase may get hit with nearly the full phase to phase voltage.

 

[bernardgreen]

Indeed and I'm convinced a disconnector used to be an 'on load' disconnecting device and was always twice the size of an isolator.

I am also convinced that that statement is correct. The difference is the amount of current the device can break safely without being damaged and ensuring the circuit is broken.

In a disconnector the arc that is created when the contacts open under load must not continue to burn.

 

[bsr]

I'm again confused. All I can see 'on the left' appears to be the earth (with a bit of G/Y tape). What am I missing?

There is another silver uninsulated block below the earth block. It's marked N.

I am also convinced that that statement is correct. The difference is the amount of current the device can break safely without being damaged and ensuring the circuit is broken.

In a disconnector the arc that is created when the contacts open under load must not continue to burn.

In my very limited experience of HV the following terms are used:
 Breaker interrupts current, often gas insulated
 Disconnector used to isolate for mantainance

 

[333rocky333]

We may need a referee

Kind Regards, John

No, I meant what Risteard is saying, makes late or even breaks early, sorry

 

[JohnW2]

If the nuetral disconnects before the phase conductors, then high voltages can develop between the now-floating neutral and one or more of the still-connected phase conductors.

True, but only in terms of the neutral upstream of the isolator. If the neutral is disconnected by the isolator, the loads will not be connected to that floating neutral - so I can't see that any harm can come to connected (now 'neutral-isolated') loads in the manner that rocky suggested?

Kind Regards, John

 

[JohnW2]

There is another silver uninsulated block below the earth block. It's marked N.

There is certainly something below the earth block, but I have to leave it to your superior eyesight to identify what it is

If that's intended for the neutral, ehat is the purpose of the fourth pole of the switch/isolator/whatever?

Kind Regards, John

 

[JohnW2]

No, I meant what Risteard is saying, makes late or even breaks early, sorry

Fair enough, but I'm still confused about the way in which this all started, when you wrote ...

Using a 4 pole as pictured could possibly cause 415 across any internal 240 volt equipment, if only for a fraction of a second

Any 'internal 240V equipment' would have to be connected between neutral and one of the phases. If the neutral 'breaks early' then any such internal equipment would be 'disconnected from the supply', such that there would be no PD between the L and N feed to that equipment (other than due to stray capacitance etc.).

What am I missing?

Kind Regards, John

 

[SUNRAY]

I am also convinced that that statement is correct. The difference is the amount of current the device can break safely without being damaged and ensuring the circuit is broken.

In a disconnector the arc that is created when the contacts open under load must not continue to burn.

Some years ago I got the job of correcting an installation error. New 75KW chilled water pumps had been fitted with 160A isolators but the spec was for disconnectors. The initial plan was to replace the actual switch within the enclosure, it would have gone in but the wiring would have stopped the door closing. The replacement enclosure was much bigger, nigh-on 2ft high and required significant alterations to unistrut and trunking.

 

[SUNRAY]

Fair enough, but I'm still confused about the way in which this all started, when you wrote ...

Any 'internal 240V equipment' would have to be connected between neutral and one of the phases. If the neutral 'breaks early' then any such internal equipment would be 'disconnected from the supply', such that there would be no PD between the L and N feed to that equipment (other than due to stray capacitance etc.).

What am I missing?

Kind Regards, John

The thing about these sorts of units is they are made of parts: Enclosure with earth and neutral terminals, switch may be available in 2 or 3 pole (and possibly 4), add on switch modules.

If you look at the pic you'll see the terminals are numbered 1/L1, 3/L2, 5/L3 across the top, the bottom is 2/T1, 4/T2, 6/T3 but the 4th pole has no markings

The additional pole can be added to anything in the range and possibly several stacked to make 5, 6 poles etc.

When purchased as a 4 pole the 4th may be marked, typically with L4 & T4 or N. This one is clearer and shows both markings:
On larger units they often come as a kit of an actuator and any number of modules which may or may not be marked

Some of the companies I've worked for keep stocks of the parts and assemble as required.





Imagine the scenario of a 10KW load on ph1, 10W halogen bulb on ph2 and nothing on ph3 and the star point loses the neutral:

I won't bother doing the Ohms law calculations but I'm sure it becomes apparent quite quickly the pd across the bulb will be almost the ph to ph voltage, I'll go so far as predicting within 99.9%

 

[plugwash]

Any 'internal 240V equipment' would have to be connected between neutral and one of the phases. If the neutral 'breaks early' then any such internal equipment would be 'disconnected from the supply',

The neutrals of the loads may be disconnected from the supply, but they are still connected to each other!