2-phase resistance welder MCB

[craigdiver]

Hi All,

I am faced with a 2-phase resistance welder which is protected by a 63C breaker, the welder, input current, is momentarily tonging in at 120A per phase whilst welding. Although the MCB-C should take at least 5xIN, should it be designed to take this 120A many times a minute or should it infact be fitted with a 125A breaker? the breaker has become sensitive causing nuisance tripping and will need replaced with something?

Any thoughts?

Thanks

 

[davesparks]

What does the data plate on the machine say with regards power/current requirements?

My gut reaction to this is that fuses could be the answer, but consulting the manufacturer of the protective devices and getting their advice once you have the data for the machine would probably be the best way to go.

 

[Knobhead]

While the current may peak at 120A, what is the average?

BTW I’ve always used fuses for welding sets?

 

[craigdiver]

What does the data plate on the machine say with regards power/current requirements?

My gut reaction to this is that fuses could be the answer, but consulting the manufacturer of the protective devices and getting their advice once you have the data for the machine would probably be the best way to go.

we have many resistance welders with, due to their age, no specification plates. The original supply fused isolator has 63A fuses but this additional RCD/MCB combo has been added at a later date as a safety measure to prevent the operators being plated when the chop through the cables. I do know the manufacturer and will seek their advice tomorrow.

 

[Darkwood]

First port of call always is the manufactures guidelines as all Welders are tailor made and spec'd to the job they are designed to do... there is no rule of thumb here and i find it highly unlikely the mcb has got sensitive and more likely the setting of the welder have been changed to account for other issues the operator may have encountered with the welding process.

Where did you get your 120A figure from and if a meter was used has it a sufficient sampling rate to measure peak Ampage associated with inrushes of inductive loads?
I assume we can correctly say the 120A figure is primary side related and your not reading it off a display on the welder....

What is the make and model of the welder?

 

[davesparks]

Perhaps measures to prevent operators from chopping through cables would be more appropriate?

Am I reading this right that you currently have fuses and an mcb in series?

 

[craigdiver]

First port of call always is the manufactures guidelines as all Welders are tailor made and spec'd to the job they are designed to do... there is no rule of thumb here and i find it highly unlikely the mcb has got sensitive and more likely the setting of the welder have been changed to account for other issues the operator may have encountered with the welding process.

Where did you get your 120A figure from and if a meter was used has it a sufficient sampling rate to measure peak Ampage associated with inrushes of inductive loads?
I assume we can correctly say the 120A figure is primary side related and your not reading it off a display on the welder....

What is the make and model of the welder?

It is a gantry balanced long arm PW welder and as stated in my OP the 120A is primary. However was measured using my basic clamp meter to get a ball park measurement.

 

[craigdiver]

Perhaps measures to prevent operators from chopping through cables would be more appropriate?

Am I reading this right that you currently have fuses and an mcb in series?

unfortunately easier said than done, the spot welder is suspended on a balancer from a trolley on a h-beam gantry and the bundle of cables/cooling circuit pipes are around 12m long (and the operators are careless oafs at times!).

 

[craigdiver]

Am I reading this right that you currently have fuses and an mcb in series?

yes, discrimination goes, 400A low voltage switchroom - 100A MCB in DB - 63A fused isolator - 63A MCB/30mA RCD combo - welder

 

[davesparks]

Was the 63A MCB added in as an after thought to the original installation?

 

[Darkwood]

It is a gantry balanced long arm PW welder and as stated in my OP the 120A is primary. However was measured using my basic clamp meter to get a ball park measurement.

Apologies missed the primary bit and I would ignore the clamp meter it probably has a poor sample rate to see the peak ampage unless it has specific setting to peak hold and a documented high sampling rate.... I have a few clamp meters and only one of them can log the peak inrush of inductive load.

As for the cabling a suitable Energy track with cables to suit would be one method or some form of insulated busbar track.. but if like my customers they don't like investing money in the short term to avoid the costs of long term maintenance.

The idea the supply cables can be damaged and may be hanging is asking for trouble if I see the setup correctly .... putting an rcd on it is not the method to resolve this and an rcd on a welder is tempting tripping issues from the start a redesign of the cabling system is needed.

Any pics and i could offer a suggestion to guard the cabling.

 

[craigdiver]

Was the 63A MCB added in as an after thought to the original installation?

yes, a shiny enclosure containing a mechanically connected MCB/RCD. Thinking of it I might just replace the MCB/RCD with a 2 pole RCD and rely on the 63A fuse links for overload protection, any thoughts?

 

[davesparks]

I think I see the problem here, an installation which was most likely designed and installed perfectly well has received an 'upgrade' in the name of safety and now it is unfit for the job in hand.

Are you sure it is the mcb dropping out and not the RCD if they are linked ?

 

[craigdiver]

but if like my customers they don't like investing money in the short term to avoid the costs of long term maintenance.

Any pics and i could offer a suggestion to guard the cabling.

Clamp meter - yes, pile of uncalibrated cack, but all i've got for rough amps measurements.

We have many gantry welders like this, the control system/water chiller units are mounted behind the gantries then 3 different electrical cables plus flow and return coolant hoses plus, as you rightly stated no budget to improve, only repair.

Good old British manufacturing!

 

[craigdiver]

Are you sure it is the mcb dropping out and not the RCD if they are linked ?

Yes, the MCB trips without tripping the RCD, but when RCD trips it takes the MCB with it. just the MCB tripping.

 

[craigdiver]

I'll take a some photos tomorrow and post so you can cringe at the setup!

 

[davesparks]

In a way that is good, it strongly suggests the problem is the fact that an MCB has been installed where it shouldn't be.

 

[Darkwood]

Sounds ideal for an energy track system ... if it ever seems a plausible option either add to thread or PM me and ill suggest some ideal systems where they may even come out and do a free quote and select all the correct gear for you ...

I still stick quotes in and ask them to review the past 5yrs maintenance costs for the existing including Electrical, coolant issues etc all associated with the existing problematic set up ... I bet it will open their eyes just how much its costing them and what they could be saving.

 

[craigdiver]

In a way that is good, it strongly suggests the problem is the fact that an MCB has been installed where it shouldn't be.

my experience is that MCBs are a modern replacement to fuse links if properly rated. It appears on this installation that the design current (which I think should be the primary current whilst welding) greatly exceeds the MCB rating. Others with more experience in resistance welding may know better and I welcome their wisdom?

 

[davesparks]

I'm afraid your are wrong, MCBs are not a replacement for fuses. They are an alternative which give a level of convenience in being easily resettable.
Both fuses and MCBs have their place in this world and it is up to the person designing an installation to select the correct one for the job.

In your situation it appears that the designer selected fuses which have been doing the job fine and then someone else added an mcb which hasn't been so great.

 

[craigdiver]

In your situation it appears that the designer selected fuses which have been doing the job fine and then someone else added an mcb which hasn't been so great.

Definitely an add on, a pretty din rail enclosure between a 20+ year old isolator and welding controller.

 

[Knobhead]

Cooling water? Are they Lincoln welders?

 

[craigdiver]

I should publicly flog myself for my way off the mark fault finding. eventually traced to breakdown of insulation on welding transformer, resistance measured fine (8.3 ohms) but shorts out when meggered. PW welder 25+ years old and encapsulated transformer obsolete.

 

[Darkwood]

shorts out to what ! .... its a low impedance coil it will show dead short if meggered ... thats the way your post reads or do you mean to earth?

 

[Darkwood]

Cooling water? Are they Lincoln welders?

Just plug topped 4 of these replacing some 50yr old American brands .... the old are still the best for durability

 

[craigdiver]

shorts out to what ! .... its a low impedance coil it will show dead short if meggered ... thats the way your post reads or do you mean to earth?

as you quite rightly state that does not read very well, shorting to earth

 

[Robotstar5]

We had many hundreds of resistance welders in BIW (body in white) at Longbridge and very few problems on primary side, it was the secondaries that took the hammering, some of the failures were quite impressive due to the current involved .
Generally fed them with motor rated fuses, 100M160A etc.