Wiring 3 terminal AC Transformer to two terminal control board

Thread Starter

Smucoy

Joined May 19, 2024
13
Likely a rudimentary question for this forum. I have a new security system control board that shipped with a 16.5VAC Class 2 transformer 40VA that has three terminals (grounding prong), but the control board which it is to be wired to only has positive and negative terminals (no ground). Is it safe to not wire the ground terminal on the transformer?
 

Ian0

Joined Aug 7, 2020
10,235
As the transformer has an AC output then wiring it to positive and negative terminals is a bad idea, because positive and negative terminals imply a DC input.
 

Ian0

Joined Aug 7, 2020
10,235
Class II implies that it is double-insulated and needs no earth connection. Class I has an earth terminal.
 

Thread Starter

Smucoy

Joined May 19, 2024
13
Ok, thanks! What is the third (middle) terminal for on the transformer? I assumed it was ground wire connection as there is a ground prong in the plug. Clearly I am a software weenie and not a EE :)
 

MaxHeadRoom

Joined Jul 18, 2013
28,820
Likely a rudimentary question for this forum. I have a new security system control board that shipped with a 16.5VAC Class 2 transformer 40VA that has three terminals (grounding prong), but the control board which it is to be wired to only has positive and negative terminals (no ground). Is it safe to not wire the ground terminal on the transformer?
Is this similar or is a wall-wart type transformer PS? Do a continuity check to see where the supply ground is connected to.
Normally the transformer frame.
 
Last edited:

Ian0

Joined Aug 7, 2020
10,235
Ok, thanks! What is the third (middle) terminal for on the transformer? I assumed it was ground wire connection as there is a ground prong in the plug. Clearly I am a software weenie and not a EE :)
The third pin on which side? Mains side (primary) or secondary?
 

Thread Starter

Smucoy

Joined May 19, 2024
13
Continuity shows the ground prong is connected to the middle terminal.

Model is MGT1640P. The 16.5 VAC 40VA Class II Power Supply with Screw Terminal Output and Auto-Resetting PTC Fuse from MG Electronics provides power for devices requiring 16.5 VAC, 40VA. It features a three-prong grounded plug, three-screw terminal output, and an auto-resetting PTC fuse.
 

Thread Starter

Smucoy

Joined May 19, 2024
13
So my question is whether I need to connect the GND terminal to an earth ground, or can I leave the GND terminal unused if the outlet it is plugged into is properly grounded.
 

MisterBill2

Joined Jan 23, 2018
19,366
What information is TOTALLY MISSING is about the "ground pin" and the three terminals.
A reasonable guess is that the ground pin is associated with the mains connection.
What is totally missing is an explanation of the "three terminals." Are they associated with the mains power connection side of the transformer?? Or associated with the 16 volt secondary? Is the output connection two screw terminals, or three??
 

Thread Starter

Smucoy

Joined May 19, 2024
13
Here are the specs I know on the power supply (transformer)
■ Input voltage: 120VAC 60Hz
■ Rate current: 2400mA
■ Output voltage: AC 16.5 V +/- 5%
■ No load voltage: AC 19.5VMax
■ Exciting current (Io): 130mA Max. at input 120VAC 60Hz without secondary load.
■ Insulation test: 500 VDC 100MΩ Min. between input plug and output terminal and case.

Three screw connection output terminals. Two outside terminals are AC and the middle terminal is GND. The alarm company sent this power supply for the control board. It replaced a previous OEM power supply that was only two terminal (no ground). There is no place on the control board or the enclosure to connect a ground wire. It seems my choice is to either not connect the ground output terminal since the original design used a two prong power supply, or to makeshift my own ground connection to the control board metal enclosure. They have since told me I can wire it up without using the ground terminal. Is that reasonable?
 

MisterBill2

Joined Jan 23, 2018
19,366
Use the two terminals that are not the ground terminal! If the board/module did not require a ground connection from the power source before, it still does not require a ground connection.
The transformer provides the isolation so there is not, and was not, any shock hazard.
 

Thread Starter

Smucoy

Joined May 19, 2024
13
thanks Mr Bill2.
There is a separate EGND terminal on the control board adjacent to the ring and tip phone line terminals which is nowhere close to the AC input terminals. I assume that exists for wiring POTS phone line connection which I am not installing. The Installation manual just says to wire to existing Household earth ground Wiring. Interestingly, it was not connected on the previous (old) system installation that did have POTS phone connection.
 

MisterBill2

Joined Jan 23, 2018
19,366
If any additional "ground" connection was not needed with the previous transformer, why wold it suddenly be needed now,with the replacement transformer??
There has been no mention about anything else changing.
 

Thread Starter

Smucoy

Joined May 19, 2024
13
I think these are two separate concerns.

1. Whether the ground output terminal on AC power supply should be wired to a grounded location. Was advised by the installation company that it did not need to be. Think the conclusion is it is safe to only wire the two AC terminals. The ground prong connects the outlet common earth ground to power supply chassis.

2. The security system control board EGND terminal wired to earth ground. Unclear whether this is for all installations, or just when connecting phone lines (presumably introducing another power input). My setup will not have phone lines connected. The installation manual simply advises connecting household earth ground wire to the metal enclosure using a bolt and nuts as grounding electrode, and then wire between that grounding electrode and the EGND terminal on the board. Trying to figure out what the EGND terminal is for if the AC input is already grounded and there are no other wired connections with power input (i.e. POTS lines). Perhaps for resiliency safety from surge protection since there are wires running from the control board throughout the house to various keypads and sensors? If recommended to connect the EGND, will I need to run a dedicated wire back to breaker box earth ground to be sure everything is bonded?
 
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