“LED signs are safe due to the low voltage”, a slogan often heard or read in advertisements, might be misleading. Besides high-current hazards, and not just with circuit shorts, hazardous high voltages may be present in the “low-voltage” LED circuit, depending on the power-supply type and construction.
In some cases of remote control, this hazard can even lead to the destruction of components. This discussion stems from some regulatory rules for the LED power-supply circuit designer. But let’s start from the beginning, because this problem surfaced in the last decade. Low-voltage sign applications aren‘t new, and LED neon signs have been used in signage for nearly 20 years. With “classic” low-voltage power supplies – those with a bulky core-and-coil transformer at mains frequency to step down the 115V to 12 or 24V – everything was fine.
But then, as modern semiconductors and electronics evolved, “switching type” power supplies debuted, and they’ve become sturdy enough for 24/7 operation in the last 5-10 years. As discussions now include energy savings and wireless-communication safety, several regulations have been implied by authorities, such as:
- Power supplies for lighting fixtures and signs must have a minimum efficiency of 95% or higher.
- Power supplies must not generate high-frequency interference to wireless communication.
Core-and-coil LED power supplies often no longer meet requirement (1), and (2) has never been an issue because no frequencies higher than mains-line frequency are generated in the circuit. The modern, switching types meet (1), but require costly and bulky additional components to meet (2). The operating principle of “switching” a voltage very quickly (30,000 to 1 million times a second) means the stepdown transformer for the same power rating can be very small and low cost compared to the “classic” types.
However, switching electricity very quickly generates high-frequency/radio-frequency (RF) currents, which must not leave the power-supply box/circuit to meet requirement (2), because the currents cause interference in nearby or remote electronic equipment. To prevent high-frequency currents from leaving the power supply, filter circuits must be used to block the high frequency on the mains and output leads of the power supply. These filter circuits block the high frequency by inductance, and the remaining currents that pass the inductance are shorted to ground by capacitors. The filter capacitors are connected directly between the mains line and ground. These capacitors not only conduct high frequency, but also a small amount of the mains frequency. As long as the capacitors can derive the RF currents to a grounded conductor, everything is fine.
The National Electrical Code (NEC), Article 600.7, requires electric signs to be grounded. But the wording doesn‘t call for the power supplies to bear a ground connection, too, in case they‘re the double-insulated/reinforced insulation type.
Such types of power supplies come with only the line and neutral terminal or wire on the mains side, but no ground connection. Most of the small “walldog“ power supplies are the double-insulated type and come only with a two-prong plug.
Back to our requirement (2): But what‘s done with the RF filter capacitors if, in a double-insulated power supply, there‘s no ground connection to which the filter capacitors could be connected? Then, they are usually connected to the output side. And that‘s what creates the trouble.
The capacitors not only feed the (very weak) RF currents into the output line, but they also superimpose capacitive currents from the mains line on the output. Capacitive mains currents, if not drawn (the output is not directly connected to ground, or at best, insulated), will put the full voltage (or half, if a second capacitor to neutral forms a capacitive voltage divider) to the output side. Yes, you read that correctly: The low-voltage output of a switching power supply can bear hazardous line voltage to ground!
The legal limit as “safe touching voltage“ is 50V, which is often exceeded, but, in case of an accidental connection to ground, the capacitors have to, per regulation, limit the current to a maximum 0.5mA per unit.
The low current prevents shocks if the output is touched, but the high voltage present can do severe damage to sensitive electronics connected between output and ground. Who cares, some readers might claim, but here‘s the point: Even if the LED circuit and modules are insulated, the insulation strength usually is dimensioned for the extra-low safety voltage (below 50V). But now this insulation has to withstand the mains voltage.
A possible breakdown of the insulation in the LED low-voltage circuit usually does no great harm, because the current is too weak to cause a fire. A real problem exists if the LED neon sign lighting system employs multiple circuits and a control bus system. LED circuits that operate on extra-low voltage aren‘t required to have the data/control bus insulated from the DC input, and so, in most cases, the control input is galvanically connected.
However, if you connect multiples of Class II power-supply-fed LED lamps with remote control to a common control bus, the outputs of the power supplies are connected together, at least a single pole. But there is not only the control voltage/signal present, but the mains voltage fed back into the control connection through the power supply. The high voltage can and will damage the control system.
Further hazards exist; because the interconnected control lines all carry the line voltage, the weak currents (of up to 0.5mA for each power supply) add up.
So, more than once I accidentally touched such control wires (which should carry only 5-10VDC) and got a bad shock. The total current of multiple power supplies leaking current on the output not only shocked me, but destroyed the (protected against electrostatic damage) lighting controller, including some data-line drivers hidden in vaults between the control center and the fixtures. In one case, the damage was estimated at roughly $500,000 – simply by not selecting a properly grounded power supply!
So, when planning a sign, or selecting the components, always choose a power supply with a grounded line input. But, this doesn‘t insure the output is free of spurious, high-voltage components. I used a low-voltage power supply (cheap Far East version) with a grounded, three-pole plug on an expensive, lighting-control test instrument, and it had the same problem. The ground pin was simply left unconnected inside the power supply, and the filter capacitors were connected to the output, as it would be in a double-insulated configuration. I got zapped and now use an old-style, core-and-coil supply. You wouldn‘t expect that from a high-quality, test instrument brand. The only safe way is measuring if the output of a low-voltage power supply is really low voltage – also to ground!