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#217442 - 08/06/16 12:53 AM Solid State Relays and electrical isolation
Potseal Offline
Member

Registered: 02/23/13
Posts: 187
Loc: Saskatchewan
Recently I was asked to replace a solid state relay on a PCB for a Siemens fire alarm system device. The device is a HTRI-R module which controls two sets of fire doors through a solid state relay (on the module's PCB) which are held open with magnetic door holders powered by a 120VAC circuit. I replaced the relay and tested the module with a Siemens DPU which tests the module but cannot verify if the relay is functioning properly. In order to see if the fix worked I placed the module back in the field in the same location it was removed from. The repaired module failed to power the mag door holders. The co-worker who originally replaced the module and requested that I replace the module's relay believes the 120V circuit that powers the mag door holders through the relay somehow damaged it. If that's true and I replaced the relay properly (good solder connections) is it possible there can be greater damage beyond the relay to other components on the module's PCB that the DPU could not detect? Or would a solid state relay generally provide enough electrical isolation from the 120V circuit to prevent further damage? Visually the relay did not show damage (ie burnt, deformed, etc).

BTW, I bench tested the relay that I removed. The relay has 2 coils and one each N/C and N/O terminals. I tested the relay with a proper power supply applied to the coil and it failed to change state of the N/C and N/O. Therefore I assumed it must be damaged.

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#217443 - 08/06/16 01:21 AM Re: Solid State Relays and electrical isolation [Re: Potseal]
Potseal Offline
Member

Registered: 02/23/13
Posts: 187
Loc: Saskatchewan
Man this is bugging me... New PCB relay, replaced module tests good with DPU, place it back in the field making the same connections as the module that is currently working properly - what could be the problem? The relay's N/C contacts should be closed unless the Fire Alarm Control Panel signals the module to change the N/C contact's state... Yet, once the module is activated the fire door's mag holders are not powered (which shouldn't really matter because the N/C contact should be closed regardless). If the module's PCB circuit is damaged beyond the relay that I replaced then how could it be the cause of making the N/C contact to open? The co-worker who worked on this problem initially went through 3 new HTRI-R modules before the problem went away. He claims he could smell a burning odour each time a module failed. I think he was hoping they could be salvaged by replacing the relays but there appears to more going on than a simple relay failure.


Edited by Potseal (08/06/16 01:23 AM)
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#217446 - 08/06/16 09:36 AM Re: Solid State Relays and electrical isolation [Re: Potseal]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
I am confused.You say SSR then you talk about coils and contacts. A SSR uses triacs to control the line side and LEDs control the low voltage side. Is there also a mechanical relay in there?
I do know highly inductive loads with a big reverse EMF spike will blow them if you do not have arc protection and they can be troubling on 3p wye with 208 line to line loads.
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#217448 - 08/06/16 10:22 AM Re: Solid State Relays and electrical isolation [Re: Potseal]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
If it is a mechanical relay that is switching the power to the door holder and the relay modules are failing, I suspect the door holding magnets may be drawing too much current. The relay might be working but the power draw might be overheating the pcb traces.

That might explain the burnt electrical smell.

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#217449 - 08/06/16 10:28 AM Re: Solid State Relays and electrical isolation [Re: Potseal]
Potseal Offline
Member

Registered: 02/23/13
Posts: 187
Loc: Saskatchewan
Nope, it's my confusion. I do not have the exact information but it is a relay similar to this one:

http://www.digikey.com/product-detail/en/panasonic-electric-works/DK1A-12V-F/255-2059-ND/1004014

Therefore, a small mechanical relay. Nonetheless, if this relay is apart of the module's PCB and experiences a high spike in current on the N/C contact side without causing any visual damage to the relay can it still create problems on the coil side and beyond to further damage the function of the circuit board?

It seems so simple - if the module removed from service tests "good" with the Siemens DPU, then the only thing that could be wrong is the relay that is used to release the fire doors incase of a fire alarm, which the DPU cannot verify. Yet, with exact relay replaced and the module put back into service the fire doors do not stay open which means power is not going through the new relay to power the magnetic door holders. Strange.
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#217450 - 08/06/16 10:32 AM Re: Solid State Relays and electrical isolation [Re: LarryC]
Potseal Offline
Member

Registered: 02/23/13
Posts: 187
Loc: Saskatchewan
Quote:
If it is a mechanical relay that is switching the power to the door holder and the relay modules are failing, I suspect the door holding magnets may be drawing too much current. The relay might be working but the power draw might be overheating the pcb traces.

That might explain the burnt electrical smell.



Never thought of that. So the damage to the boards could be minor but enough to change the N/C contact to open.


Edited by Potseal (08/06/16 10:33 AM)
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#217452 - 08/06/16 02:22 PM Re: Solid State Relays and electrical isolation [Re: Potseal]
twh Offline
Member

Registered: 03/11/04
Posts: 892
Loc: Regina, Sask.
Maybe the fault melted another connection on the circuit board.

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#217453 - 08/06/16 07:02 PM Re: Solid State Relays and electrical isolation [Re: Potseal]
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9012
Loc: Estero,Fl,usa
Have you cut open one of the bad relays and looked at it. The problem might be obvious. I am not sure I understand tho. What fixes it, a new relay or a new board? The driver that picks the relay may be shorted so it is picked all the time. If you can get the cover off the relay you should be able to see that.
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#217454 - 08/06/16 07:24 PM Re: Solid State Relays and electrical isolation [Re: Potseal]
LarryC Offline
Member

Registered: 07/05/04
Posts: 775
Loc: Winchester, NH, US
Let's step back and start from the beginning.
1) system worked at some time in the past.
2) system stopped working.
3) Someone replaced relay board three times until system started working again.
4) You replaced relay on relay board, tested it on a test fixture, which said relay board is OK. Relay board was installed in system and system does not work.

Is this the sequence of events?

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#217455 - 08/06/16 07:25 PM Re: Solid State Relays and electrical isolation [Re: Potseal]
sparkyinak Offline
Member

Registered: 07/08/07
Posts: 1286
Loc: Alaska
Just a thought. Being its a FA panel isn't anyone concern that by "field modifying" it is voiding the panel listing or is it going to get reevaluated by the listing organization?

What about just replacing the board in question? I know it sucks to throw out so much for so little. However that's what a customers signs on for with such high tech equipment. One if my biggest gripes about techie equipment and the industry in general.
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