Insulation diagrams according to 60601-1 – Part 2: Preparation

In the last article, some basic concepts of insulation diagrams were explained. In this article, we will go step by step through the creation of an isolation diagram.

1. Survey of the system context

The first question to be clarified is which device and in which environment the insulation diagram should be created? Which standards apply to the device? Are there any special requirements according to DIN EN 60601-2-XX? In what environment will the device be used, e.g., an ambulance or a hospital?

As an example, the following is an insulation diagram for a mains-operated electrocardiography device (ECG) of protection class II with a type BF applied part and control via a USB connection (SIP/SOP).

2. Information and boundary conditions

The second step is to actually gather the necessary information. To create the insulation diagram, the following questions must be clarified:

• What protection class does the device have? Possible protection classes are:

• How is the housing constructed? Does the housing contain conductive metal parts? Are these insulated or connected to the protective conductor or an applied part?

• Up to what altitude and at what air pressure should the device be operated?

The operating altitude must be known, as the altitude above sea level is related to the operating air pressure. Lower air pressure and thinner air place greater demands on the air gaps. At low air pressure, arcing occurs more quickly because there are fewer molecules in the air.

Unless otherwise specified, medical electrical devices are designed for altitudes up to 2000 m. If the device is to be used at an altitude > 2000 m, the minimum clearance distance is multiplied by a factor according to Table 8 of DIN EN 60601-1.

• Classification of application parts

This is usually clearly evident from the application and is taken from DIN EN 60601-1 and the special specifications according to DIN EN 60601-2-XX. Possible classifications of the applied parts are:

• Determination of the degree of pollution

In what environment will the device be operated?

There are 4 levels of pollution:

Pollution degree 4 is unacceptable for insulation that represents a protective measure.

• Overvoltage category of the mains connection

There are four overvoltage categories for the peak voltage of the mains connection, listed in Table 10 of DIN EN 60601-1. Overvoltage Category II is generally used unless the manufacturer specifies a different overvoltage category. Here, the respective special requirements of DIN EN 60601-2-XX must be checked. Other overvoltage categories apply, for example, to permanently installed devices.

• Overvoltage category of the secondary circuits

As a rule, overvoltage category I applies to secondary circuits as long as they are derived from a mains connection of overvoltage category II.

• Special features of the device

Is the device powered by a wall adapter? What power outputs might occur? Are there any special environments?
All of this should be clarified beforehand.

• Does the device consist of several parts?

For example, the device may be part of a medical electrical system. What is the layout, and what modules are there? An explanation of the modules and their properties should be provided in the insulation diagram.

• Tracking Index (CTI)

If the material tracking index is unknown, IIIb is used. The material tracking index is either specified in data sheets or can be obtained from the manufacturer (e.g., the printed circuit board).

3. Create insulation diagram

The following must be drawn:

• All touchable conductive housing parts
• All patient connections
• All signal inputs and signal outputs
• The supply connections
• Internal circuits that are isolated from other parts of the circuit (e.g. high voltage, intermediate circuit voltages, …)
• Safety-relevant components such as protective resistors, relays or fuses may be included.

A simple block diagram for the isolation diagram might look something like this. As already mentioned above, it is a mains-powered device with a Type BF applied part, e.g., an ECG, and a SIP/SOP, e.g., a USB port for control.

4. Draw insulation distances

In the example diagram, the routes have already been drawn and marked with letters.
In general, the following routes should be marked:

• Routes from the circuit parts with different operating voltage to the housing (in the example B, C, D)
• Distances from the application part to the mains connection (in example G)
• Distances between application parts (in example I)
• Distances between application parts and other circuit parts isolated from them (in the example F, H)

and then the section A for opposite polarity in the power supply.

5. Tabular formulation of requirements

The insulation distances and test voltage values can then be determined in tables. The following general requirements apply:

• For patient protection 2 MOPP between application parts and mains connection.
• At least 1 MOPP (8.5.2.1) between applied parts at highest mains voltage.
• Between housing and applied parts of type F 1 MOPP at highest mains voltage.
• Between different application parts of type F 1 MOPP at highest mains voltage.
• 1 MOPP for 250V* is required between application parts and protective conductor connection.
• At voltages above 42.4V_AC or 60V_DC In the secondary circuit, the operator must also be isolated with 2 MOOP.
• Operator protection with 2 MOOP against mains connection

*) The 250V instead of 230V is considered the highest mains voltage.

These are the most important requirements in summary. Please refer to your copy of the standard and check the information when creating an insulation diagram.

5.1. Determining the insulation distances

This is best done in tabular form according to the requirements listed in the standard.

a) No protection against secondary voltage required because < 42.4V_AC or 60V_DC
b) Route I is designed with 2 MOPPs because it cannot be guaranteed that a device with correct separation is connected to the USB port.

5.2. Determining the voltage to be applied

The values for this can be obtained from Table 6 (Table 7 if referred to in Table 6).

a) No protection against secondary voltage required because < 42.4V_AC or 60V_DC
b) The voltage of 2500V results from the fact that the sections E + F or F + H must not be less voltage-resistant than the section G or I lying parallel to them.
c) No test for voltages < 42.4V_AC according to Table 6.

5.3. Determining the clearance and creepage distances

The corresponding values for patient protection can be found in Table 12 of the standard.

The air distances for operator protection against the power supply are shown in Table 13.

The clearances for operator protection against secondary circuits are given in Table 15.

The creepage distances for operator protection are shown in Table 16.

a) No protection against secondary voltage required because < 42.4V_AC or 60V_DC
b) The voltage of 2500V results from the fact that the sections E + F or F + H must not be less voltage-resistant than the section G or I lying parallel to them.
c) In Edition 3.0 of 60601-1, Table 11 still existed for parts of opposite polarity in the power supply. This has been deleted in Edition 3.1.
d) The values for clearance and creepage distances are conservative, as interpolation is permitted. Thus, for a 230V mains voltage, a creepage distance of 2.3 mm could also be assumed for 1 MOOP.

6. Review by experts / colleagues

Before the diagram is sent to an approval authority, experienced colleagues should conduct a review. It's rare for a review to fail to uncover something.

Final tips

It has proven useful to consider the following points:

• The insulation concept should be reviewed by the approval authority early in the project.
• It's convenient to create the diagrams with a single tool. Schematic editors are well suited for this; we use Altium Designer, for example.
• For medical devices covered by 60601-1, creepage distances must also be maintained on the inner layers of printed circuit boards. This can be circumvented by performing a thermal cycling test according to 8.9.3.4 of 60601-1, but this requires considerable effort.
• All relevant standards, especially the Special Specifications 60601-2-xx, should be reviewed for references to the insulation diagram. These often contain requirements, such as that an applied part must be defibrillation-proof.

If the topic is interesting and relevant to you, I recommend you to register for our Newsletter We will soon publish a brochure on the topic Insulation diagrams according to 60601-1By subscribing to our newsletter, you will be informed as soon as it is ready.

And if you need help creating an isolation diagram or are interested in an external reviewer, you can always contact us contact to record.