Machine Safety

[CEDEng]

For the following...can anyone refer me to good article or other source of understandable info?

What is your solution for machine-enclosure door and panel safety? This is NOT referring to operator safety, or moving parts and guarding.

I am referring to the "rear door" of large machines and panels. Do you use a limit switch or safety switch of some kind? What is the function of the switch?
Imagine a machine with multiple doors and removable panels.

This is not an ESTOP situation - it's a maintenance question. I've seen it done many ways, but am having trouble finding a definitive "best practice."

Thanks, all, for your input.

 

[Timbert]

I'm having a little problem understanding your question. You say, "This is NOT referring to operator safety, or moving parts and guarding." Are you asking how to handle this situation if the machine is a threat to itself but not to anyone else?

:?

 

[cadpoint]

My belief is that your looking for the term, micro switch or microswitch.

Some of the major players in US are Honeywell, and GE, Digi-Key and a few others
and in Europe is Pizzato.

 

[K8MHZ]

I'm having a little problem understanding your question. You say, "This is NOT referring to operator safety, or moving parts and guarding." Are you asking how to handle this situation if the machine is a threat to itself but not to anyone else?

:?

I think he means that only maintenance personnel would be using the rear door, not the operators of the machine, and the enclosure contains no moving parts.

 

[cadpoint]

Micro switch's are by no mean limited to only moving part machinery.

There are E-stop micro switch's on the doors of the cabinet power supply's at work.

There are totem pole signal alarms on gas vault's they can be programmed to signal anything from good to low
pressure and even faulty ajar cabinet doors.

Any new use of micro switch's will have to be broadcasted and understood by both the employee's
and outside contractors. We just went through this at work!

Micro switch's can be programmed into PVC - ladder logic or chained together or stand alone.

I'm not that in depth with other various NFPA literature but you can got look at NEMA's new
Utility safety: NEMA publishes 2015 Electrical Standards & Products Guide.
[B]http://www.utilityproducts.com/a...-electrical-standards-products-guide.html[/B].

Realize that is a trade listing publication of NEMA members and a sprinkling of bought ads.

 

[cadpoint]

Still digging!

Granted this might be old hat to you, but for those reading along and not always
exposed to different things.

Just a thought for what might be required when you finally get your design!

You are going to have to take the characteristic's of the equipment that your going to
apply your design to and take into account the types of NEMA Enclosures.
https://www.nema.org/Products/Documents/nema-enclosure-types.pdf

This is the compete list of 1 through 13 enclosure types for both Non-Hazardous Locations,
and Hazardous Locations!

 

[Besoeker]

My belief is that your looking for the term, micro switch or microswitch.

Some of the major players in US are Honeywell, and GE, Digi-Key and a few others
and in Europe is Pizzato.

Obviously, I don't get involved with NFPA very much.
So just a bit of information on one of the common methods used for safety interlocking here.

Sometimes called trapped key interlocks.

Castell or Fortress locks are often specified for industrial control panels.
They are usually very robust (read expensive) brass affairs and fitted to the panel doors and the panel supply circuit breaker.

When the panel doors are closed, that releases the keys from their locks. These, in turn fit a key exchange box which then releases the previously trapped key for the breaker. When that key is removed from the box, the panel door keys are then trapped. The breaker key allows the breaker to be closed whereupon its key is trapped.

It may sound a bit convoluted but it works. It requires you to do things in the right sequence.
Can be a bit of a challenge if you need to do live testing.......

 

[iceworm]

... Sometimes called trapped key interlocks.

Castell or Fortress locks are often specified for industrial control panels.
They are usually very robust (read expensive) brass affairs and fitted to the panel doors and the panel supply circuit breaker.

When the panel doors are closed, that releases the keys from their locks. These, in turn fit a key exchange box which then releases the previously trapped key for the breaker. When that key is removed from the box, the panel door keys are then trapped. The breaker key allows the breaker to be closed whereupon its key is trapped. .......

That would be what we call a "Kirk key". "Kirk" is a trade name, however they are the only ones I have ever seen. Definitely old school - definitely work well. The new ones I have seen look just like the ones I saw 50 years ago

http://www.kirkkey.com/default.aspx?Page=Products

ice

edit - I see on their brochure they are called "trapped key interlocks" Apparently Kirk is not the only outfit in the world that makes them.

 

[CEDEng]

Yes - the Kirk Key is something like what I'm asking. I'll try to phrase my question another way!

We build machines whose dangerous electrical parts (transformers and contactors and so forth) are inside of a "box" - an enclosure. Some enclosures are "store bought" while others are fabricated in-house. These enclosures have doors on hinges, panels with slotted "holders," and other various means of attachment to allow access to the innards of the thing, not normally seen by the operator.

Sooner or later, SOMEONE is going to open one of those doors, be it maintenance or service or troubleshooting or...

Now, when someone opens the door - while the machine power is ON - what should happen? And how is it implemented usually?

Hope that helps clear things up a bit...thanks, all, for your good tips already.

 

[petersonra]

Yes - the Kirk Key is something like what I'm asking. I'll try to phrase my question another way!

We build machines whose dangerous electrical parts (transformers and contactors and so forth) are inside of a "box" - an enclosure. Some enclosures are "store bought" while others are fabricated in-house. These enclosures have doors on hinges, panels with slotted "holders," and other various means of attachment to allow access to the innards of the thing, not normally seen by the operator.

Sooner or later, SOMEONE is going to open one of those doors, be it maintenance or service or troubleshooting or...

Now, when someone opens the door - while the machine power is ON - what should happen? And how is it implemented usually?

Hope that helps clear things up a bit...thanks, all, for your good tips already.

There are various ways to handle this.

A common one is to have some kind of interlock with the door so that the door cannot be readily opened with the power on.

Another way is to put a padlock on the door that the operator does not have a key for.

Sometimes in lieu of a padlock a seal is used.

Putting a switch on the door is a bad idea IMO. It implies in some way that opening the door with power on is an acceptable practice.

Generally speaking though, the most common way to restrict access is to require a tool of some sort be used to remove the panel or open the door.

Most places if an unauthorized person opens an electrical panel they are subject to some pretty draconian disciplinary measures. That tends to discourage random people from opening up such panels. Presumably, persons who are authorized to open the electrical panels have been trained in either shutting off the power first or opening it in a safe way.

 

[Besoeker]

That would be what we call a "Kirk key". "Kirk" is a trade name, however they are the only ones I have ever seen. Definitely old school - definitely work well. The new ones I have seen look just like the ones I saw 50 years ago

http://www.kirkkey.com/default.aspx?Page=Products

ice

edit - I see on their brochure they are called "trapped key interlocks" Apparently Kirk is not the only outfit in the world that makes them.

What I like about them is that they are purely mechanical. There are no electrical parts to go wrong and a power failure, total or partial, doesn't affect their operation.

 

[iceworm]

Yes - the Kirk Key is something like what I'm asking. ...

We build machines whose dangerous electrical parts (transformers and contactors and so forth) are inside of a "box" - an enclosure.

These enclosures have doors on hinges, panels with slotted "holders," and other various means of attachment to allow access to the innards of the thing,

Now, when someone opens the door - while the machine power is ON - what should happen? And how is it implemented usually? ....

I've seen this done two ways:

One door:
The key is trapped in the feeding CB lock. The CB must be turned off to remove the key. The key is then released and used to unlock the door. The key is trapped in the door lock until the door is closed and locked.

Multiple doors:
A key block is set up with one key (from the feeder CB) releasing all the keys for the doors. The CB lock key is trapped in the block until all the door keys are back in the block

Multiple power disconnects/multiple doors:
A key block is set up with one key that will release all the door keys. Turning the one key to the release position opens an aux contact that opens all the power sources (contactors, shunt trip CBs)

As bes said, troubleshooting with live voltage measurements is difficult.

As Bob said, train them up - that's what qualified means.

Usually these types of interlocks are installed where the risk is more than normal. 600v and below is generally considered normal risk. MV controls (example: big VFD drives) - not so normal. The design goal is to require the qualified personel to have to take an extra step to get the doors open.

ice

 

[CEDEng]

Thanks again, all, for the great responses.

I think we're making progress: A keyed system that requires multiple steps to get through. Follow-up -

Now that the multiple steps have been completed, a (hopefully) qualified individual proceeds to open the door.

At this point (assume>600V), what should happen - and how is it implemented?

We all (here - at this shop) agree that it would be ideal if power were magically unhooked when the door opens. The (hopefully) qualified troubleshooter will then use various nefarious means to get around that, if powered troubleshooting is required.

I am looking for a reasonable method with which power can be removed. If it's a "doorswitch" of some type - what type? Does it operate a (for the purpose of over-simplification) a giant contactor that simply unhooks all the power clear back to the last cable entry? Assume an enclosure with six doors, for instance.

There must be a "general" way this is done - is there no "standard" that says "Protect your doors with method X" ?

 

[petersonra]

Usually >600V stuff is locked up either in a room or inside a fence.

I don't see any reason to over think this or do something out of the ordinary.

 

[iceworm]

... Now that the multiple steps have been completed, a (hopefully) qualified individual proceeds to open the door.

At this point (assume>600V), what should happen - and how is it implemented?

We all (here - at this shop) agree that it would be ideal if power were magically unhooked when the door opens.

I am looking for a reasonable method with which power can be removed. If it's a "doorswitch" of some type - what type? Does it operate a (for the purpose of over-simplification) a giant contactor that simply unhooks all the power clear back to the last cable entry? ...

CED -
Please go back and read post 12. There are no magic door switches. The "Trapped Key" required to release the door keys, either comes from the lock on the feeding CB, or activates an aux contact that trips the power feeders. If the keys for the doors are free to open the doors, then the power is off.

... The (hopefully) qualified troubleshooter will then use various nefarious means to get around that, if powered troubleshooting is required. ...

Yes, us t-shooters, as well as being nefarious, are sneaky, underhanded, and devious. Yes we will get around it as needed.

... I am looking for a reasonable method with which power can be removed. ...

Read Post 12 again.

One source: Use a trapped key lock that can only be removed when the CB is off.

Multiple sources/multiple doors ....

Welcome to the world of being the "Engineer of Record"

ice

 

[CEDEng]

It's the "ordinary" I am trying to define!

It's not actually >600V. I used that, because I want to secure 48V as if it were 600V. This is in response to a mandate from our largest customer - that all enclosure doors and panels (>48V) be interlocked and protected in the "standard" way. Which is escaping me! What standard? What way?

I know this sounds elementary - but after building these machines for a variety of companies for many years, I can confidently say that no two companies (or engineers) do it alike. I am looking for a standard that says it "must do THIS."

Since standards are many and vague, I am instead polling the community (here) to see what y'all do as "standard." Surely your machine panels and doors are interlocked SOMEHOW so the maintenance man cannot just open it up and commence work. Assuming he has used the proper tool ("locks..."), what - then - happens when he opens the door, and machine power is ON?

I don't mean to "overthink" it - but finding a concrete solution has proved elusive!

 

[petersonra]

It's the "ordinary" I am trying to define!

It's not actually >600V. I used that, because I want to secure 48V as if it were 600V. This is in response to a mandate from our largest customer - that all enclosure doors and panels (>48V) be interlocked and protected in the "standard" way. Which is escaping me! What standard? What way?

I know this sounds elementary - but after building these machines for a variety of companies for many years, I can confidently say that no two companies (or engineers) do it alike. I am looking for a standard that says it "must do THIS."

Since standards are many and vague, I am instead polling the community (here) to see what y'all do as "standard." Surely your machine panels and doors are interlocked SOMEHOW so the maintenance man cannot just open it up and commence work. Assuming he has used the proper tool ("locks..."), what - then - happens when he opens the door, and machine power is ON?

I don't mean to "overthink" it - but finding a concrete solution has proved elusive!

The reason it has proved elusive is that you are demanding a standard that does not exist. This problem is handled in any number of ways, any of which might well be perfectly safe.

IMO, there is no reason in most cases to interlock access panels at all. Just make sure they use a fastening system that requires the use of a tool. If someone goes to the trouble to take out 4 or 6 or 8 screws to get at something in direct violation of company rules, nothing would have stopped him.

A door is a little different, but again it depends on just what the door is for. A lot of junction boxes in factories. You going to put some kind of interlock on everyone of them?

Incidentally, NFPA79 (standard for machines) REQUIRES that there be a way to bypass disconnect switches that are interlocked with a door.

 

[Besoeker]

Putting a switch on the door is a bad idea IMO. It implies in some way that opening the door with power on is an acceptable practice.

[/QUOTE]

Or maybe not so bad.
A typical clause from a LV variable speed inverter drive:

3-pole door interlocked off load fuse-switch, padlockable in the off position.

Door interlocked means that you can't open the door with the power on.
You have to turn it off to be able to open the door.

The padlock allows someone, say a fitter, to turn off the isolator and ensure that it can't be turned back on until he removes his padlock.
In fact, it's fairly common here to have hasps that allow multiple padlocks to be fitted so that all those potentially affected can protect themselves.

My guys all carry their own and I have a few spares in my car glovebox to cater for anyone else we might need on site.

 

[CEDEng]

Iceworm - my apologies - one of your most educational posts must have appeared at the very moment I was composing the reply, so I missed some of your relevant info until now. Thanks!

Meantime - I know it seems goofy - but what the customers demand, it's what they're gonna get. Which is interlocks on every door and panel.

I do plenty of servicing in the field for these things - so I am familiar with all the many and popular ways of working around that which is designed to prevent my demise.

I was beginning to wonder - weeks ago - if in fact there was NOT a standard to address this. I could not find one, nor info, thus my post here. I suspected there wasn't a concrete answer, like ESTOP info and so on (which, while vague, is at least existing...).

Here's an example: I use an industrial limit switch on the door to do BLANK. Customer states: "That is inadequate. It must be a safety switch. That's the standard."

So began my quest...

 

[petersonra]

Here's an example: I use an industrial limit switch on the door to do BLANK. Customer states: "That is inadequate. It must be a safety switch. That's the standard."

So began my quest...

You cannot use a standard limit switch to perform a safety function.

What you are asking about is not a safety function.

There is nothing in any code that says you should have or must have a limit switch on any door of an electrical enclosure, regardless of what is behind the door.