“I have a question about grounding. Some equipment comes with a three prong grounded AC cord and some does not. I never seem to run into significant ground loop problems with the two prong (ungrounded) variety, but I frequently seem to have problems with the three prong type. I am aware of the safety hazards involved in simply lifting this safety ground, but I wonder: why can some equipment be safe without this ground pin, and given how much nicer that stuff seems to play in my studio why can’t more equipment be made this way?”
Well, we don’t have space for an Electrical Engineering course on power supply designs, but suffice to say that in a two pronged AC cord the neutral wire (connector) serves as ground inside the device. That is not to say it completely replaces the dedicated ground, but when designed properly the device is perfectly safe. The specific details of this really aren’t important unless you are designing equipment. But it is important to be aware that in devices with three pronged (grounded) power cord the ground plug must be connected to a working safety ground in order to be effective (read safe). Lifting this pin, while sometimes solving ground loop problems, will compromise your safety. We’ve done many tips on this subject matter so feel free to search the inSync archives for more info.
It is true that in some situations ungrounded equipment will be less prone to ground loop problems. This kind of goes hand in hand with not having that extra ground. But is it better? The question becomes, what is the upside to having equipment with a grounded AC supply as opposed to without? We already know the advantage to the ungrounded supply. What’s the advantage to the grounded supply? For this your inSync editor consulted with audio guru, and former Sweetwater Employee, Mark Phillips. Mark is currently the Technical Manager at ARN Consultants (this is Rupert Neve’s design company). His (edited) comments follow:
If you have ground noise you don’t have a good ground. The problem stems from the fact that the electrical safety need only have the capacity to open the circuit breaker, it is not required to be a Technical Shield/Ground Path. The safety can have several ohms of “connection” resistance, especially if there is conduit involved. The safety also does not care about multiple paths to ground. These can spoil a shield. (Ed – A common workaround for ground loop problems is lifting the shield at one end of all audio cables (see telescoping shield), the problem of noise coming into your gear over the ground pin is much more difficult to solve.)
While a safety ground can be a hindrance in some setups it can actually be a huge benefit in larger setups where ground loops can build up. It makes an excellent star ground attach point. If the service safety ground is clean you can get great results with proper lifting of signal shields. If the service is not clean, you still have simple access to your own Technical Star Ground. A properly configured and grounded setup can be cleaner and have fewer problems than a system involving mostly equipment without the separate safety ground. Bottom line: it can be more work, but in the end the results can be better.