Category B - Relay or no?

I currently design small panels - mostly motor control panels (1, 2, 3 motor) and the owner of the company is asking us to cut back on costs.

One of the items he wants to cut out on is our standard category B Estop - which is usually a simple E-stop with control relay, a reset button and a control power on pilot light.

He would like us to go down to a single Push-pull Estop and no relay whatsoever - just run the power through the E-stop contacts.

Our machines have motors that are completely enclosed. We are only looking at Category B. We don't usually use more than 3A of current.

Does anyone see an issue with this? It just seems strange to not have any kind of MCR relay.

The only reason to use an MCR is if you have too many circuits that need to be isolated from one another, but powered down from a common E-Stop. Most PBs will accept least 4 sets of contacts, some a lot more than that and most contacts are rated as A600, so Make 6, break 60, 10A continuous. So as long as you are OK with either having everything on the same control circuit, or running that many control wires up to the door (or wherever you put that E-Stop button), there is no reason not to do it.

What he wants is not allowed.
you will have to look at NFPA 79, electrical standard for industrial machinery.
going through a push pull contact defeats safety.
no machine may automatically start upon the reset of power.

I don't have access to a copy right now.
if I am wrong, someone please correct me.

Risk assessment

Rson said:

I currently design small panels - mostly motor control panels (1, 2, 3 motor) and the owner of the company is asking us to cut back on costs.

One of the items he wants to cut out on is our standard category B Estop - which is usually a simple E-stop with control relay, a reset button and a control power on pilot light.

He would like us to go down to a single Push-pull Estop and no relay whatsoever - just run the power through the E-stop contacts.

Our machines have motors that are completely enclosed. We are only looking at Category B. We don't usually use more than 3A of current.

Does anyone see an issue with this? It just seems strange to not have any kind of MCR relay.

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Safety Category is not a function of "Owner's" budget mandate.

As an engineer, you specify on required function FIRST.

Understood, that if the previous design was just over kill assortment of safety parts, then you may have wiggle room.

……...BUT do you want your name on paradigm reduction in safety performance on the equipment?


Have the "Owner" give you a pie chart of the overall cost of the entire sale price of the project, and then highlight the slice that represents the benefit of dropping the level of safety.

i don't know if there is a danger of harm to people working on the machine. for me i do not like the idea that if you have a power outage that when the power is restored that the motors will start. i will not let any thing start back up on its own. at least a 3 wire latch!

wancues said:

i don't know if there is a danger of harm to people working on the machine. for me i do not like the idea that if you have a power outage that when the power is restored that the motors will start. i will not let any thing start back up on its own. at least a 3 wire latch!

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The motors would not start back up on power loss. They use a start/stop latch for the start circuit that would de-energize and require someone to push a start button to re-start.

I agree the control power would not be de-energized on power loss - control power would be active upon power start-up, and a motor could start if a contact welded shut.

The first thing I would recommend is to get your liability insurer involved. Have a good talk with their underwriter, your boss and yourself. Not having the correct safety configuration can expose your company to major liability if something goes wrong and somebody gets hurt.
I have talked to many safety engineers about this very issue, from my understanding thee-Stop button cannot be defined as an E-Stop unless it is monitored by an approved safety relay. If there is no safety relay it’s just a stop button. All safety relays must require supervisory level personal to reset them, in fact I have install safety relay system that require a key or password to reset.
You must also train the operators not to use the E-Stop as a normal stop for the equipment.
I would rather error on the side of safety, the cost of a few pieces of extra hardware over somebody getting injured is a no brainer to me.

I have run into many E-Stop’s configured using a single line system. This makes no real sense to me because all E-Stop devices have at least 2 sets of contact so the only difference is the cost of the wire. As for labor you have to wire them either way. I always use 2 wire monitored design on all E-Stop system it’s better to have extra safety then come up short if something happens. At least I can say I made every effort to keep it make is as safe as I can.

I have recently observed small panel shops not using monitored safety relay at all just a single line through the E-Stop button. The reason I was given was that they experienced nuisance trips do to the panel environment. I would fix the environmental problem before eliminating the safety relay.

There are many ways to deal with this it’s all about risk management.

As for the MCR relay that is a different matter altogether. If you are using an MCR as part of a safety system then you must wire 2 relays in series controlled individually from the safety relay and they must have force guided contacts.
I worked in one shop that had a policy the each panel had to have 2 disconnects in the feed line. Both had to visible from the panel. To work on the panel you had to have both locked out tagged out.

Yes safety is a big deal and should be carefully considered on every job.
I know this is a long reply but we all must make safety a priority all the time. The first time you mess up could be the last thing you ever do.