Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
(OP)
Hi Guys,
I am in Australia where we use IEC standards and we don't have any requirements for UL listing of protective devices.
We have a class 30 overload application with a soft starter with line contractor. What sort of branch protection should I be using?
The distributor is telling me that we should use a magnetic only (you guys call it an MCP) breaker upstream and let the soft starter do overload protection for the feeder cable, motor cable, and contactor.
What do you think? Will the thermal protection in the soft starter protect the feeder cable and motor cable, even if the soft starter is set up for a class 30 overload?
What about the contactor? can the soft starter be relied upon to do thermal overload protection for it too?
Cheers
I am in Australia where we use IEC standards and we don't have any requirements for UL listing of protective devices.
We have a class 30 overload application with a soft starter with line contractor. What sort of branch protection should I be using?
The distributor is telling me that we should use a magnetic only (you guys call it an MCP) breaker upstream and let the soft starter do overload protection for the feeder cable, motor cable, and contactor.
What do you think? Will the thermal protection in the soft starter protect the feeder cable and motor cable, even if the soft starter is set up for a class 30 overload?
What about the contactor? can the soft starter be relied upon to do thermal overload protection for it too?
Cheers





RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Here in North America it could not be done at all. UL and CSA both ceased allowing MCPs to be used on power electronics years ago, even by the starter mfrs. There were just too many problems.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
We will have high speed fuses on the starter which will achieve type 2 coordination for the starter, I'm more concerned about the cables and the contactor.
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
In fact this is a perfectly valid setup in IEC 60947.4.1 for a combination starter. I'm pretty sure I saw a starter in there which can was just magnetic + contactor + overload.
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
The soft-starter overload could protect the cables and contactor. The question would be if you sized the cables and contactor so they would be protected by the overload curve you're using.
In North America, UL allows an MCP to be used in a listed combination and CSA allows it in a built-up panel or listed application. For UL, the limitations are not just on power electronics, you can't build any kind of motor control panel unless the MCP and starter are a listed combination.
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Lionel has a good question. Changes the answer significantly.
If there is question in my mind, I'm a fan of laying out the cable damage curve, fuses, and the overload curve on a TCC log-log chart. I don't recall ever seeing a damage curve for a contactor. However, most everything I use is NEMA rated, not IEC.
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
When I say 'line contactor' I mean a contactor in series with the starter, on the line side of the soft starter (there will actually be two contactors, for safety)
The issue I have with using the soft starter to protect the cable is that the starter we have does not allow us to select a trip class in accordance with IEC 60947-4-1. The motor thermal protection is very configurable however I can't easily present it in the form of a tripping curve that I could plot the cable damage curve against.
I know most vendors starters (Siemens, Schneider, Rockwell) and most electronic overload relays for DOL starts allow a selection of a tripping class, but this one doesn't.
At it's maximum setting this starter will allow 12xLRA for 120s, which is way way over a Class 30 overload scenario.
The additional complication here is that this is a MCC replacement where the field cabling to the motors will be reused, hence we can't upsize it, and chances are it's already had a hard life as it has been in place for the last ~20 years.
Has anyone visualised a soft starter's trip characteristic against the cable damage curve in the case where the starter is doing overload protection for the cable
And, yes, regarding the contactor the only information I have is a continuous current rating (which is about 1.3x the AC-3 rating) and the short circuit protection that would be required for Type 1 and Type 2 coordination scenarios.
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
But you're hopefully going to set it to 6xLRA for 30s (Class 30), or something more specific to the motor specs (locked rotor current and locked rotor time). Assuming that the starter will allow these parameters?
And hopefully this cable is sized correctly to the motor, if not, then you're going to be in a bit of trouble.
Pretty much this.
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
True, but as I said, no longer for VFDs and Soft Starters, as of I think 2014, maybe early 2015. They stopped allowing NEW submissions like that even longer ago, around 205 or 6 IIRC, but were not forcing "grandfathered" designs to be changed change. That's what ended recently; even older grandfathered designs are no longer acceptable with MCPs, the breakers must be T-M.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
It would be alot easier, albeit less configurable, for the starter to utilise a standard trip class.
It would also make me feel alot more secure about someone messing with the settings later on and compromising the cable protection. I know alot of field commissioning/programming types who would modify soft starter trip characteristics which would affect the cable protection without knowing it.
Anyway I think the main thing I wanted some assurance on was the use of the soft starter to protect the cable when the soft starter's trip characteristic is not one of the standard trip classes.
Even if the motor FLA matches the cable ampacity if the starter's settings are wrong then the cable may not be protected in certain areas on the curve. I dont think this is possible if you can only select up to a class 30 overload, but I haven't proved this myself.
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
http://www.hub.aucom.com/manual/1056-710-04840-00-...
Thanks all for your time I really appreciate it. I have been on this forum a long time and I have read alot of posts from this thread's respondents and I definitely am very grateful for your advice
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
http://www.hub.aucom.com/application-notes/1086-71...
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
If so, and you call Aucom to get guidance on how to set up that protection, keep in mind though that in doing so, you may exceed the thermal ratings of the soft starter itself, meaning you may need to have over sized it to use it at Class 30. At a miminum, Class 30 implies that the motor will be able to handle Locked Rotor Current of 600% for 30 seconds. The basic sizing of that soft starter will be LRC for 10 seconds, so without contacting them for what adaptation it might be capable of, you may be looking at sizing the soft starter at 3x the motor FLA to attain this capacity.
One of our esteemed members of this forum, MarkE, is the founder and inverter of the Aucom soft starter. Too bad he has been quiet here lately, he could likely help. But I know he would be upset to hear that nobody at Aucom stepped up to help you, have you called them directly?
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
http://www.lmpforum.com/forum
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Thanks all for your responses so far
Thanks jraef. I don't know that my motor is rated for Class 30 OL, no. I will be getting the motor data tomorrow and I will have a look at it's design characteristics and go from there. I have tried to contact Aucom directly but I am always forwarded on to our local distributor, who are not Aucom.
Update on this project is as follows...
Background that was missing before is that this project is replacement of DOL conveyor starters with soft starters - 30kW 55A motors direct coupled to grain conveyors.
The starters are AuCom EMX3 already oversized - nominal current 105A and thermally they have a nominal FLC of 60A at 50 degrees C at a duty of 4.5xFLC for 30s with 570s off time (105A:AC53b 4.5-30:570)
The technical risk I am trying to manage, and it comes up all the time in DOL to RVSS projects, is the potential for the soft starter to not be able to produce enough starting torque to get the loaded conveyor away.
I am not versed at all with conveyor calculations and I am not confident (without a very clear how-to) of doing conveyor calculations to determine the minimum breakaway torque required and relating this back to the motor's reduced voltage torque characteristic. Also the project does not have the budget for me to school myself on these calcs. Dont get me wrong I would love to learn how to do these calcs accurately but it just doesn't fit into the bounds of this project budget or timing.
So the current limit on this starter is 600% of FLA, but the starter has an instantaneous trip of 6x nominal rating = 600A. It seems the only thing I can do is to design this system to be able to do as close to a DOL start as I can - as I know the conveyors get away fine at the moment with a DOL start.
So, if I wanted to get the starter to do 700% or 800% FLA on a current limit start, 800% would be 440A which is under the starter's instantaneous trip level of 600A. However, this would require me to 'fake' the motor FLA setting in the soft starter, which would adversely affect the motor and cable protection. This could be fixed by installing an external overload relay.
So really the question I have to answer is can I justify the installation of an external overload relay on this starter.
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
ANYWAY, even if you could set the starter for 700 or 800% current limit, it's pointless. Your MOTOR is what determines the maximum torque it can supply. The soft starter can only REDUCE that torque. Locked rotor torque on a standard Design B motor will be about 160% of FLT, which from a dead stop will equate to around 600% of FLC. You cannot make the motor draw more than that no matter what controls it. So if you are concerned as to not having enough torque, you have the wrong motor, soft starter or not.
PEAK torque from an AC induction motor takes place at the Breakdiwn Torque point, which typically occurs at around 80% speed shen started DOL. the only way to take advantage of that peak torque capability is to use a good Vector Drive, which can "force" the motor into delivering that briefly at any time.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
A question;
Breakdown torque at 80% speed implies 20% slip, or a rotor frequency of 12 Hz.
Can you program most drives to begin ramping from 12 Hz, and if you do, will you be able to accelerate at near breakdown torque? I realise that the drive may have to be oversized.
Thanks Jeff.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
jraef
I would be interested in seeing the specification which prohibits using a MCP with a VFD. I am particularly interested in the CSA specification. Can you provide me with any references?
Thanks
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
I can't speak to CSA, but in general they tend to follow each other's rules.
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
We can only use MCPs as part of an approved assembly. We cannot use MCPs for field built controls.
If we can find a VFD that is part of an approved assembly with an MCP, we may use it. We cannot use an MCP for either a magnetic motor starter or a VFD for field built assemblies.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
NEC 1996 reads essentially the same.
However, no factory assembly requirement. Still one would have to get the assembly field listed - that definitely could be a problem if the NRTLs don't like it - and plenty of money.
I'm a bit surprised NRTL agencies won't list ASDs with a mag-only CB. Their usual concern is only that the device not start a fire - not that it actually works. Self-destruction is only an issue if it causes a fire.
Interestingly, 2014 NEC 430.130.A specifically allows instantaneous CBs with ASDs. Still have to be listed.
ice
Harmless flakes working together can unleash an avalanche of destruction
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
And going back to the previous comment from waross,
breaker + starter having an overload (ie a contactor and overload) = combination motor starter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Listing or approval is covered in section 2
2-024 Use of approved equipment (see Appendices A and B)
(1) Electrical equipment used in electrical installations within the jurisdiction of the inspection department shall
be approved and shall be of a kind or type and rating approved for the specific purpose for which it is to
be employed.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
We have all seen instances of an installation that was passed by one AHJ and failed by another. Most of us have seen installations passed by an AHJ that maybe should not have been passed.
I fondly remember one local inspector and a number of imported ticketing machines that required local approvals. All our inspector looked at was the serial number. If the number was correct, the machine passed.
Then one day the customer changed out a faulty machine without letting me know. The number was wrong. The inspector just drew a line through the number on the paper work and wrote in the correct number and passed the machine.
Passing inspection in one jurisdiction does not guarantee that an installation is universally approved.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
RE: Cable protection for Class 30 overload application with soft starter - MCP or thermal magnetic MCCB
Default is 10 seconds which is the equivalent of a class 10 overload.
For high inertia loads, you need to first have a motor capable of a long locked rotor current, (mining duty motors are commonly 40 seconds) and set parameter 1B accordingly.
Best regards,
Mark.
Mark Empson
Advanced Motor Control Ltd