Interlocking

Hi,
As I'm relatively new to this area can someone please give me a definition
and a basic example of Interlocking.I have a hazy idea already but a few
comments from more experienced people would be appreciated.


Thanks.

1. To connect or be connected by fitted parts.
2. To be connected so that neither part can be operated without the other.
3. Electrical circuit used to block an action until certain conditions have been met.

Which industry do you want ?

I'm sure the interlocking for the control system in a nuclear power station might be different from that found on a third world packaging machine.

Thanks for the response:
So am I right in saying that Interlocking is simply a 'If - Then'
situation implemented in hardware/software?

Yes but it tends to be negative logic; i.e. If not condition A then allow condition B.

This interlocks 2 outputs A and B so that only one can be on at a time.

 

      B

-----|/|-------(A)

 

 

      A

-----|/|--------(B) 

Quick example comes from motor control.

Case one
a two speed motor.
Both speeds cannot run at same time.
So you put a switch in slow speed start circuit that is controlled by fast speed. And you put one in fast speed circuit controlled by slow.

Case two
Motor that has foward and reverse
Again cannot do both at same time
Have same switch setup as that above
In addition as required by NEMA (I beleieve ?) the relays are mechanically interlocked such that only one can shut.

Case 3.
COmpressor or other machine that needs oil
Put swithc from oil pump that will not let compressor start until after oil pump has oil pressure up. Commonly a pressure switch but you can use contacts from the pump start circuit also.

Howzat ??

Play with it and come up with some drawings on how you would wire it up. Let us take a look at em. YOu will get more ideas and suggestions and different ways to to the 3 above than you ever imagined.

Dan Bentler

Inerlocking

Hi imagine you were 3 people x,y,z each with a lock (padlock) and each of you is to lock one safe(money), now look here x will not access the money (open the safe) unless y and z have opened respectively,This is inter-locking
Its like you have got to feed material on 3 anded belts X,Y,Z down stream respectively now logicaly belt z starts first folowed by y and the lstly x but what wil happen if you started with x ? this is inter-locking its just a safety measure.

Interlociking is a safety strategy to prevent harm or damage. A simple example of an interlock is a microwave oven, if the door is opened the microwave stops. Another simple example is your a cars ignition, it wont start if the car is in gear. In some cars you can not put it in gear unless you are pressing on the brake.

As was mentioned with motors you may need to interlock mechanically or electrically. Here is an example of 3 motors that are interlocked, Motor 1 must be started first, then Motor 2 can be started then Motor 3

Here is another example of a reversing motor control that is electrically interlocked so if running in either direction can not be started in the other direction (the starters are usually mechanically interlocked too).

Ron

You're a tough act to follow.

Do you do all of tose drawings?

best regards.....casey

Violate NEC ?? ie switch neutral ??

rsdoran said:

Here is another example of a reversing motor control that is electrically interlocked so if running in either direction can not be started in the other direction (the starters are usually mechanically interlocked too).

Click to expand...

Ron

In your drawing you show the overloads between the coil and the other side of the line (control power).

QUESTION If this were 120 V
(I know your dwg does not callout voltage)
AND
other side of the line were the neutral

WOULD NOT this be switching the neutral and therefore non compliant with NEC ??

Dan Bentler

Hi Ron,

How do you do animate those drawings? they are very good.

I have to agree with Dan on switching neutrals, we recently built a panel to a design from an American company for use in the UK and they had done the same thing, we built it as drawn but I was not too happy about the arrangement. Is this a general practice in the USA? what are the NEC regs on this? I din't think the EU and UK regs are all that clear. It is not an accepted practice in the UK except in mines and quarries where the 110v ac circuit is wired as 55V-ground-55V and then both sides must be switched.

John.

Tell the truth I did not draw these pictures (but I do draw and animate pics) and technically not sure "NOW" if it is against NEC or not...it didnt use to be, it was the standard way of doing it at one time.

Looking at some things though I think the "proper" form NOW is to put the OL in series with the control "hot" like this:

leitmotif said:

Ron

In your drawing you show the overloads between the coil and the other side of the line (control power).

QUESTION If this were 120 V
(I know your dwg does not callout voltage)
AND
other side of the line were the neutral

WOULD NOT this be switching the neutral and therefore non compliant with NEC ??

Dan Bentler

Click to expand...

Dan,



This was a common practice many years ago, and it has stubbornly persisted despite being frowned upon by the NEC. Some old-school maintenance electricians just can’t make the paradigm shift. Old design NEMA starters typically had the thermal overload unit (which was a eutectic heater element) integrated into the starter. Instead of selecting a separate overload to attach to the starter, the properly sized eutectic heater elements were installed into the starter. These starters were factory wired with the overload contacts in series with the coil as shown in the example drawing and it was sold as an assembled unit. The connections were typically made with preformed jumper bars and not amenable to rewiring. The practice was to specifically show the integrated overload contacts on the electrical drawings. It didnt really matter though as long as a three wire magnetically held control circuit was used. Interupting the neutral line dropped out the coil, opening the hold-in contact, and depriving the coil of line power.


Most of the time now it is more common to use electronic overloads or bimetallic overloads. These units are specified separately from the contactor whether it’s a NEMA or and IEC contactor. On the very rare occasion that I use an integrated eutectic heater overload, I still draw it on the neutral side, because that’s the way the starter is delivered. But when I am using a separate overload where I wire the connection to the OL contact, I place the contacts on the line side of the coil.



90% of the starters I do now are soft starters (or VFDs) and I don’t even bother showing the overloads as they are internal, I represent the integrated unit as a box on my drawings.


I liken this to another prevailing US practice, that of placing the ground prong on 120V outlets on the bottom. The ground prong was intended to be on the top, but almost immediately electricians started installing them with the prong on the bottom, and now ask almost anyone, they will say a properly installed outlet is actually upsided down.


ie, it should be
o
| |

not
| |
o

Alaric,

You say the ground plug 'was intended to be on top'. Why should it make any difference? I just looked at the one under my desk. The ground is on the top.